I didn't know what to do...

Last night I was staying at my cousins house and 2 of my male friends also stopped there. As it got late we all lay down to sleep. One of the males lay behind me which i was fine about because we were just innocently lying next to each other watching TV. When we were lying down I started to feel him thrust against the back of my body particularly my bum... I didn't want to turn around and say no because I felt rude but I felt extremely uncomfortable... I used to be fine with stuff like  his happening but now I'm not. I feel uncomfortable and really emotional about the whole thing... What do I do?!