Lena Dunham makes Jimmy Fallon incredibly uncomfortable

Lena Dunham dropped by "The Tonight Show" and made Jimmy Fallon incredibly uncomfortable when she brought up something her mom recently said to her.

Her mom was commenting on Lena's boyfriend of two years, musician Jack Antonoff. She said she loved Jack ... but she always thought Lena would marry Jimmy Fallon.

LENA: "She said it like we had a previous relationship, like you were my first boyfriend and things didn't work out, but she thought we'd get back together. Not like I had like 50 magazines covers of you up in my bathroom."
JIMMY: "I mean ... that's hilarious, that's hilarious."
LENA: "I knew this would make you ... a little embarrassed."
JIMMY: "A little bit embarrassed."
LENA: "It's okay."

Jimmy should've been prepared for that, though -- when Lena stopped by his old show in 2012, she professed her love for the late-night host. She was "obsessed" with him in high school.

Jimmy quickly changed the subject to 'Girls' -- not the general topic, but Lena's award-winning HBO show. The season finale airs Sunday.