I did something bad with my friends ex!! Please help!?

Hey guys, I just graduated highschool and I am going to be a freshman in college just so you know I'm not super young or anything. Okay, so back in March I went to a party with my friend Cassie. Cassie was dating a guy named Jake from May-October 2010 and then they had a nasty breakup and she said she is COMPLETELY over him and hates him. At the party Jake was there but they didn't talk. Cassie left early and I stayed behind. There was alcohol at the party and I said I would only take one sip but of coarse made a bad decision and drank more (I know it's terrible!). Jake and I talked a lot (I was a bit tipsy) and he kissed me and we had sex. We talked about it later and we both felt bad about it but he said I was beautiful and he liked it. I liked it too (even though I was drunk) but I felt terrible about it. Unfortunately, Jake turned on his charm like always and we did it again, and again, and again. We've been having an intimate relationship for about 3 or so months but I feel absolutely terrible about it (you could say "friends with benefits"). I am way to scared to tell Cassie about this! Should I tell her? She said she was over him anyway. Please don't judge me for this, I just need help. I don't want to tell anyone about this because they will judge me and stuff. I really like Jake though.