"I thought, 'Oh my God'," she said in this week's People's cover story.
"There was this sort of very excited, teenage-y kind of moment."
The former "Friends" star, 47, who says she's learned to embrace her appearance over the years, insists she feels her best when she's healthy and strong.
"It's funny, it's a really quick transition from not a care and now all of a sudden, we've got to really be mindful of what we put inside our bodies," she added.
"And how we sleep and take care of ourselves. You can get away with a lot in your 20s.