confused with best friend

My best friend and I have been sleeping with each other since April.  We have never really acknowledged it and ignored what happened the next time we see each other. We hung out this weekend and my other friend noticed he was catering to me and asking if I was ok (time of the month). We started making out. I asked who else he was sleeping with and he said his "main side chick". He implied he wanted to do more than make out and I told him no. I guess he got mad because he told me to get off of him. So I did and I went to sleep. Now I have no idea what I should do or tell him. I'm not going to sleep with him if he's sleeping around. I'm afraid our friendship is ruined now.