Why is he doing this???

Ok so my ex and I hadnt tlkd 4 like 3months, he acted like he hated me he always does that when we arnt dating, anyways he just asked me out about a week ago and I told him no cus I already have a boyfriend and then he said o ok, I told him that I did like him a little and that we could b friends and he said ok. I told him that if me and my boyfriend ever breakup he is an option, anyways so the next day him and his friend wanted me 2 hangout with them, I asked if I could bring my boyfriend cus I was with him at the time and they said ok. So we hungout and my ex kept staring at me and showing off, but overall things went ok. Then the next day I called him and tlkd 2 him and he told me that he still loved me. So I was like ok, and he told me that he wouldnt b mad if I stayed with my boyfriend and that I should do what makes me happy. So I was like wow he really cares about me, and then he told me that his friend called me a wh0re and he did 2. I was like what why??? He said that he was mad about something, anyways his friends are all being aholes 2 me now and are spreading rumors about me, and now 1 of his friends just called me a wh0re. I don’t know why they are doing this when they know im not, and how do I get them 2 stop??? Please help!!!