HELP PLEASE!!

Okay so I use to be friends with this girl, and she started to drift away from me when she got a boyfriend (which is normal I guess) but then she decided that she didn't want to be friends with me because of my eating disorder because it made her upset. Which I thought was selfish and it really hurt me. Then one night I was hanging out with her boyfriend which isn't unusual because we hang out a lot. But this time it was different and he kissed me and we ended up have if sex. I have no feelings for him, I did it out of anger toward her. But now I really bad and I think I should tell her?