Okay, so here is some backstory.

I was cheated on by my ex-girlfriend. She was sending pictures to other guys, flirting and touching with other guys behind my back. My close friend, C,, before me, got cheated on by this same girl, and he told me about it. Being dumb, I still ended up dating her anyways because at the time, she was giving me a lot of attention I never really received. He told me so.

So on Valentine's Day, she began dating the new kid in my grade, let's call him D. D and I were pretty good friends already, so he told me about it. I began to tell him about what my ex did to me and C in the past. D scoffed this off and said I was just jealous, even though I broke up with her and am with someone else. D was cussing me out, and I just said that it was fine, and it was whatever. They were happy as can be.. or so D, C and I thought.

Today, I see on my ex's story that she became single again. I was confused, and obviously since she was my ex, I wasn't going to ask her. I went to D at the end of the school day. He said that he found out that she was hooking up with some dude that she was friends with for a while. I asked who, and it was one of the guys she sent nudes to during my relationship with her. I told him that I told him that this would happen, and he was really upset at me, starting cussing at me again and stormed off.

I was shocked, since I really was looking out for him when they started dating. AITA for this?