This is kind of weird but...?

This is going to sound really strange, but I have a friend who has a belly on or something. She keeps poking my belly on, and me being extremely ticklish I can't help but laugh and not fight back. Just yesterday some of my other friends and I went to her house for a sleep over and she got my other friends to hold me down so she could tickle my belly on. I really don't like being tickled, and it was kind of embaring. How should I tell her I feel violated and I want her to stop?