ET's Lauren Zima caught up with Siwa at WE Day California, airing Aug. 17 on NBC, where the 14-year-old star -- who appeared on two seasons of Dance Moms -- revealed she visited Miller on Wednesday.
"I actually went down to the hospital to see her," Siwa shared. "She's not good, but she luckily is in good spirits. She's not 100 percent on what's going on right now, which is not the bad thing, she's definitely in good spirits, but it's so heartbreaking to see what's going on. It's so, so heartbreaking."
Siwa definitely has a lot of compassion for Miller, and says that the 51-year-old reality star has been holding back her tears.
Abby Lee Miller from 'Dance Moms'
Abby Lee Miller from 'Dance Moms'
UNIVERSAL CITY, CA - AUGUST 11: Actress Abby Lee Miller attends the Teen Choice Awards 2013 at Gibson Amphitheatre on August 11, 2013 in Universal City, California. (Photo by Jason Merritt/Getty Images)
LOS ANGELES, CA - APRIL 13: Abby Lee Miller arrives at the Logo NewNowNext Awards 2013 held at The Fonda Theatre on April 13, 2013 in Los Angeles, California. (Photo by Michael Tran/FilmMagic)
WEST HOLLYWOOD, CA - FEBRUARY 24: TV personality Abby Lee Miller attends Chopard at 21st Annual Elton John AIDS Foundation Academy Awards Viewing Party at West Hollywood Park on February 24, 2013 in West Hollywood, California. (Photo by Stefanie Keenan/Getty Images for Chopard)
SANTA MONICA, CA - MARCH 24: TV personality Abby Lee Miller attends the 3rd Annual Whispers From Children's Heats Foundation Legacy Charity Gala at Casa Del Mar on March 24, 2017 in Santa Monica, California. (Photo by Alberto E. Rodriguez/Getty Images)
LOS ANGELES, CA - MAY 14: Abby Lee Miller is seen on May 14, 2017 in Los Angeles, CA. (Photo by GP/Star Max/GC Images)
Reality TV show star Abby Lee Miller leaves at the federal courthouse after pleading not guilty to federal charges in Pittsburgh, Pennsylvania November 2, 2015. Miller pleaded not guilty in a Pennsylvania courthouse on Monday to concealing over $755,000 in income she is accused of earning while head instructor on "Dance Moms" and spinoff shows. REUTERS/Elizabeth Daley
LOS ANGELES, CA - MARCH 11: Actor Abby Lee Miller at Nickelodeon's 2017 Kids' Choice Awards at USC Galen Center on March 11, 2017 in Los Angeles, California. (Photo by Frazer Harrison/Getty Images)
SANTA MONICA, CA - MAY 30: Dance Instructor Abby Lee Miller attends the Abby Lee Dance Company LA's VIP Grand Opening at Abby Lee Dance Company LA on May 30, 2015 in Santa Monica, California. (Photo by Vincent Sandoval/WireImage)
INGLEWOOD, CA - MARCH 28: TV personality Abby Lee Miller attends Nickelodeon's 28th Annual Kids' Choice Awards held at The Forum on March 28, 2015 in Inglewood, California. (Photo by Jason Merritt/Getty Images)
INGLEWOOD, CA - MARCH 28: TV personalities/dancers JoJo Siwa, Maddie Ziegler, Kalani Hilliker, tv personality Abby Lee Miller and dancer/tv personality Mackenzie Ziegler, accept the Favorite Reality Show Award for 'Dance Moms' onsteage during Nickelodeon's 28th Annual Kids' Choice Awards held at The Forum on March 28, 2015 in Inglewood, California. (Photo by Lester Cohen/KCA2015/WireImage)
SYDNEY, AUSTRALIA - MARCH 12: Abby Lee Miller and Maddie Ziegler arrives at the 2015 ASTRA Awards at the Star on March 12, 2015 in Sydney, Australia. (Photo by Caroline McCredie/Getty Images)
CULVER CITY, CA - JUNE 14: Abby Lee Miller attends the Opening Night Party during the 2017 Los Angeles Film Festival at Culver Studios on June 14, 2017 in Culver City, California. (Photo by Kevin Winter/Getty Images)
INGLEWOOD, CA - AUGUST 24: TV personality/Choreographer Abby Lee Miller attends the 2014 MTV Video Music Awards at The Forum on August 24, 2014 in Inglewood, California. (Photo by Jeff Kravitz/FilmMagic)
NEW YORK, NY - JULY 16: (L - R) Shad Moss, Abby Lee Miller, Keshia Chante attends 106 & Park at BET studio on July 16, 2014 in New York City. (Photo by Bennett Raglin/BET/Getty Images for BET)
NEW YORK, NY - JULY 16: TV personality Abby Lee Miller visits 106 & Park at BET studio on July 16, 2014 in New York City. (Photo by Bennett Raglin/BET/Getty Images for BET)
RIDGEWOOD, NJ - JULY 15: Abby Lee Miller signs copies Of Her Book 'Everything I Learned About Life, I Learned From Dance Class' at Bookends Bookstore on July 15, 2014 in Ridgewood, New Jersey. (Photo by Jim Spellman/WireImage)
Abby Lee Miller, The FOX Teen Choice Awards 2014 arrivals (Set B) at the Shrine Auditorium in Los Angeles, California. August 10, 2014. (Pictured: Abby Lee Miller)
LOS ANGELES, CA - JULY 08: Dance Instructor Abby Lee Miller arrives at Marvel Universe LIVE! Age Of Heroes World Premiere Celebrity Red Carpet Event at Staples Center on July 8, 2017 in Los Angeles, California. (Photo by Ari Perilstein/Getty Images for Feld Entertainment)
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"She is very sick, she's very ill," Siwa said. "I don't know if it's officially announced that she has it, or she's keeping that private, but what she has is a very bad thing and you don't want anyone going through that. I was just telling her, people see her as the scary lady who yells at kids on TV, but it's not true, and she is a person first of all, and she's loving. She is caring, she taught me so many things that I'm so grateful for that it's so hard to see her go through that and being sad, and just laying there and trying to cry, but physically not being able to. She just kind of, like ... she would start to cry ... and then she would just stop."
Siwa reiterates that despite Miller's tough reputation on TV, she definitely has a softer side.
"I think people think that she is just that person on TV ... and no, Dance Moms is by no means scripted, but it is also a TV show, so you do amp it up and you do get a little mad, but behind that she is a loving, caring, nice person and she really, really is."
Miller was rushed into surgery in the early hours of Tuesday morning after several days of suffering from extreme pain in her back, arms and neck area, her orthopedic spine surgeon, Dr. Hooman Melamed, told ET after the operation. Performing a CT scan, the doctor discovered a mass infection putting pressure on Miller’s spine, then rushed her into emergency surgery to remove the infected mass.
Dr. Melamed told ET on Wednesday that Miller's cancer diagnosis is preliminary, and the results are still pending a pathology report. Burkitt lymphoma, which is a non-Hodgkin lymphoma, is a form of cancer that affects the lymphatic system and starts in antibody-producing B cells, which are part of the immune system.
“People who have been diagnosed with this have a good prognosis," Dr. Melamed said. "We’ll know more details soon."