Internal battle

There was a night out with my house mate during college. We were having a great time- she was single & was enjoying chatting to guys in the club while I, being in a relationship, was enjoying dancing either on my own or with guys if they wanted to dance. We had a few drinks while out but I know we hadn't drank that much because we were broke students. Whenever a guy would try make a pass at me, I'd politely explain I wasn't single, etc,etc. Near to the end of the night, my housemate was still chatting to this group of guys & she asked if I would like to go back to the guys' house with her for a house party. I said OK because I knew she really fancied one of the guys. I remember being handed a drink but then after that I don't have all the pieces of the picture. Everyone else must have gone off to bed because it was just me & this guy in the living room. I remember it being later in the night/ early morning & I wanted to sleep on the couch. He put all the sofa cushions on the floor - that this would be more comfortable. He was trying to kiss me as I was lying down but I was trying to turn away from him. I definitely remember telling him no, that I had a boyfriend. I can't remember if I had fallen asleep/ passed out but the next memory I have is of him pulling down my tights & underwear. I find it so hard to think of this.. because I'm constantly battling with myself over it. He wasn't rough, didn't hurt me... but wouldn't accept my "no" & if I'm being perfectly honest, from the bits I can recall- I remember actually enjoying it. That is why it is so hard- I felt so guilty after that. I felt like I had done something wrong, that I had cheated on my boyfriend. That they didn't do anything wrong if I enjoyed it. I have never told anyone about that night. I realise now that there must have been something in the drink I was given & logically, I know he was in the wrong for not getting my consent. I can't shake this feeling of shame though.. this guilt.. years later.