I Can't Stop Exposing My Friend's Belly

I think it's a fetish or something, but everytime my friend stretches her arms or lies down I can't help but lift her shirt and expose her belly to our friends. She has some belly fat and whenever I lift her shirt she doesn't have enough time to suck in her stomach like she does in pictures, so her belly looks flabby and sometimes jiggly when she's eaten a lot that day. I like how she turns red everytime and covers up instantly. She has this embarrassed fake smile all the time and avoids looking at anyone.
I do this frequently, especially after she ate because I know her belly will look fat and everyone will laugh at her. I think I like embarrassing her but I have no bad intentions. I would never do it in front of guys or something, just girls but she still gets embarrassed and tells me to stop because she thinks it makes her look bad in front of the other girls and they start making fun of her and her chubby belly. But despite that I can't stop, it's become a habit or something, like I literally did it this morning when we were in the school's bathroom with some classmates, I lifted her shirt when she was washing her hands in front of the mirror and she couldn't cover up because her hands were soapy so everyone got a good look and started giggling.
I like to do it in front of big crowds because they start laughing harder which makes her blush even more, and she gets more embarrassed when we don't know those people.
The only time I went too far was when I did it in front of some girls she hated and they hated her, because of me they found another way to bully her and started calling her fat.

Is this normal between friends or should I work on it even though I know it'll be hard?