EXCLUSIVE: Shannen Doherty says her dog was first to detect her breast cancer

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EXCLUSIVE: Shannen Doherty Reveals Her Dog Was the First to Detect Her Breast Cancer

Before her frightening breast cancer diagnosis, Shannen Doherty received a surprising warning sign.

The former Beverly Hills, 90210 star tells ET that it was actually her beloved dog, Bowie, that first appeared to detect that something was wrong.

"She would obsessively sniff right here on my right side for a long time before," Doherty tells ET's Jennifer Peros. "So, it made sense when I got diagnosed."

WATCH: Shannen Doherty Breaks Down Over Breast Cancer Battle - 'I Don't Look Past Today'

The 45-year-old actress was officially diagnosed with breast cancer in February 2015. Doherty now says that the disease has spread to her lymph nodes.

See photos of Shannen Doherty through the years:

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Shannen Doherty through the years
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Shannen Doherty through the years
WEST HOLLYWOOD, CA - APRIL 19: Actress Shannen Doherty attends Jennie Garth's 40th birthday celebration and premiere party for 'Jennie Garth: A Little Bit Country' at The London Hotel on April 19, 2012 in West Hollywood, California. (Photo by Angela Weiss/Getty Images)
LOS ANGELES, CA - FEBRUARY 13: Actress Shannen Doherty attends Shepherd Conservation Society's 2nd Annual 'World Love For Dolphins Day' at Japanese Consulate on February 13, 2015 in Los Angeles, California. (Photo by Joe Kohen/Getty Images)
PARIS, FRANCE - MARCH 13: (EXCLUSIVE COVERAGE) Actress Shannen Doherty arrives at Charles-de-Gaulle airport on March 13, 2014 in Paris, France. (Photo by Marc Piasecki/GC Images)
LOS ANGELES, CA - APRIL 09: Shannen Doherty is sighted at The Grove on April 9, 2012 in Los Angeles, California. (Photo by Noel Vasquez/FilmMagic)
WATCH WHAT HAPPENS LIVE -- Pictured: Shannen Doherty -- (Photo by: Peter Kramer/Bravo/NBCU Photo Bank via Getty Images)
LOS ANGELES, CA - MAY 10: Shannen Doherty attends STK Los Angeles' 3 Year Anniversary Party at STK on May 10, 2011 in Los Angeles, California. (Photo by Todd Williamson/WireImage)
LOS ANGELES, CA - DECEMBER 14: Shannen Doherty signs copies of her new book 'Badass' at Barnes & Noble bookstore at The Grove on December 14, 2010 in Los Angeles, California. (Photo by Jason LaVeris/FilmMagic)
Shannen Doherty arrives at the Los Angeles premiere of 'Burning Palms' held at ArcLight Cinemas on January 12, 2011 in Hollywood, California.
NEW YORK - NOVEMBER 02: Actress Shannen Doherty promotes 'Badass' at Borders Books & Music, Columbus Circle on November 2, 2010 in New York City. (Photo by Jason Kempin/Getty Images)
Shannen Doherty celebrates the grand opening of Bravada Women's Athletica at Bravada - Women's Athletica on June 17, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
NEW YORK - APRIL 01: Dancer Mark Ballas (L) and television personality Shannen Doherty pose for photos in Midtown Manhattan on April 01, 2010 in New York City. (Photo by Ray Tamarra/Getty Images)
LOS ANGELES, CA - MARCH 27: Shannen Doherty attends Perez Hilton's 'Carn-Evil' Theatrical Freak and Funk 32nd birthday party at Paramount Studios on March 27, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
LOS ANGELES, CA - MARCH 26: Shannen Doherty is seen on March 26, 2010 in Los Angeles, California. (Photo by Jean Baptiste Lacroix/WireImage)
LOS ANGELES, CA - JANUARY 16: Actress Shannen Doherty arrives at The Art of Elysium's 3rd Annual Black Tie Charity Gala 'Heaven' on January 16, 2010 in Los Angeles, California. (Photo by Jeffrey Mayer/WireImage)
LOS ANGELES, CA - DECEMBER 18: Actress Shannen Doherty arrives at Flaunt Magazine's 10th Anniversary Party and Annual Holiday Toy Drive at the Wayne Kao Mansion on December 18th, 2008 in Holmby Hills, CA. (Photo by Michael Bezjian/WireImage)
PASADENA, CA - JULY 11: Actress Shannen Doherty speaks during the 2006 Summer Television Critics Press Tour for the Oxygen Network at the Ritz Carlton Hotel July 11, 2006 in Pasadena, California. (Photo Frederick M. Brown/Getty Images)
NEW YORK - NOVEMBER 3: (ITALY OUT, NY DAILY NEWS OUT, NY NEWSDAY OUT) Actress Shannen Doherty attends the Fendi Flagship store opening November 3, 2005 in New York City. (Photo by Arnaldo Magnani/Getty Images)
Shannen Doherty during DKNY Jeans Presents Vanity Fair in Concert With a Live Performance by Alanis Morissette Benefiting Step Up Women's Network at Hiro Ballroom at The Maritime Hotel in New York City, New York, United States. (Photo by Jamie McCarthy/WireImage)
Shannen Doherty during Olympus Fashion Week Fall 2005 - Sweetface by Jennifer Lopez and Andy Hilfiger - Backstage Arrivals at The Tent, Bryant Park in New York City, New York, United States. (Photo by KMazur/WireImage)
Shannen Doherty at the HP Portrait Studio in Park City, Utah (Photo by Jeff Vespa/WireImage)
Shannen Doherty (Photo by Jeffrey Mayer/WireImage)
Shannen Doherty during The 56th Annual Primetime Emmy Awards - Fox After Party - Arrivals at Spago in Beverly Hills, California, United States. (Photo by J.Sciulli/WireImage for The jLINE Group, Inc.)
Shannen Doherty during World Premiere of 'S.W.A.T.' at Mann Village Theater in Westwood, California, United States. (Photo by Jon Kopaloff/FilmMagic)
Shannen Doherty (Photo by Ron Galella/WireImage)
Jason Priestley, Shannen Doherty and Luke Perry (Photo by Ron Galella/WireImage)
394962 09: ***EXCLUSIVE*** (Publicist Approval Required for ALL Usage) Actress Shannen Doherty poses for a portrait January 2001 in Los Angeles, CA. (Photo by Jeff Katz/Getty Images) (Clients must call or email Getty Images with EXACT frame number, client name, name/type of publication, date of intended publication and type of usage (cover, inside full page, etc) and WAIT for Getty Images to confirm publicist approval before allowing client to commit these images to use.)
Shannen Doherty, circa 1988. (Photo by Getty Images)
Actresses Shannen Doherty and Shalane McCall attend Starlight Foundation Benefit on September 22, 1988 at Ed Debevic's Restaurant in Beverly Hills, California.
LITTLE HOUSE ON THE PRAIRIE -- Season 9 -- Pictured: Shannen Doherty as Jenny Wilder (Photo by Frank Carroll/NBC/NBCU Photo Bank via Getty Images)
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She has undergone three out of eight rounds of chemotherapy thus far and says she'll do radiation once the chemo has been completed. In May, Doherty underwent a single mastectomy.

"When I got back from surgery, [Bowie] again was kind of right in this area," Doherty says, gesturing towards her right side. "Then when I had my first chemo, she would sniff my entire body up and down. And she was always protective before, but she has become this crazy, protective dog. It's hard to get close to me when she's around."

WATCH: Shannen Doherty Shares Photo of Her Completely Shaved Head

Bowie, named after the late music icon David Bowie, even sometimes sports an adorable "Security" shirt while watching out for her famous mom.

Tune in to Entertainment Tonight on Tuesday for more from our exclusive, emotional sit-down with Doherty.

WATCH: Shannen Doherty Says Breast Cancer Battle Has Made Her Marriage 'a Thousand Times Stronger'

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