What should I do??!!

So I got myself in a bit of a sticky situation and i am unsure what to do! (p.s. this may be really long but I don't know what bits to cut out haha) .
I am a 20 year old female and currently studying abroad. I am in 2nd year and I live with 3 other girls and we are all very close with another guy in our year - like every time me and my flatmates go out we invite him too, he is like the 5th member of our flat. So a few months back (probably like 6 months ago actually) I started developing feelings for this guy. However the problem is that one of my flatmates and best friends also really liked the same guy. I was the only one she told that she likes him but I didn't tell her I felt the same way about him.
So a few months ago all 5 of us went on a night out, and me and this guy were quite drunk and very flirty with each other the whole night. Later on in the night he tried to kiss me but i refused because I didn't want to hurt my best friends feelings because I knew she really liked him. As the night carried on he continued trying to kiss me but nothing happened but we did leave early together. The next day my friend was acting all weird with me and annoyed with me because i was flirting with him the night before. I told her nothing happened but she was salty with me for like 2 months after (no joke). Anyway a few weeks later we were at another party and me and the guy ended up leaving together and we had sex. My friend knew I stayed at his but we both said that nothing happened between us. After this I continued seeing him on my own and staying over at his. He told me he really liked me and that he wanted to be his girlfriend - actually he said he loved me when he was drunk but I told him he was being ridiculous because we hadn't even kissed yet!
So everything was going well then Christmas holidays came around and throughout the break me and him continued to text etc. After the break we had exams so I did not see him much, but I thought after exams we would go back to seeing each other. However after exams he stopped messaging me and it felt like he was avoiding me. So i text him because I hadn't spoke to him in weeks and asked him if he wanted to meet up with me - he didn't reply for a week!
At this point I got annoyed at him because he was ignoring me and acting as if everything before christmas didn't happen. I then told my other flatmate about the whole situation and she confronted the guy about it. He said that he thought it was just a casual thing between us but it was clear to me that it wasn't casual as he said he wanted to be my boyfriend etc. I finally met up with the guy and he didn't seem to see any problem between us and didn't think it was weird we hadn't spoken since.
Its now a month later and everytime my flatmates and him hang out I feel really awkward and it's annoying because only 1 of them knows. So I am unsure if I should tell my friend that we had sex or just never tell her, however I'm scared she will find out another way. I feel really bad about what I did but I can't change it so I don't know if there is any point in telling her because its in the past.
P.s. forgot to say the boy was a virgin also so I don't know if he just wanted to hookup with me to lose his virginity or something?!