Why is my ex mad at me?

We broke in January because she was cheating on me with my co worker. We had a argument when I fought out. We stop communicating since until July where I saw her at a event. She came and talked to me though I didn't want to talk to her. eventually we unblocked each other and became acquaintances. She would message me every so often to talk. I'm dating as well so I really wasn't into her. I had no strings attached sex with her twice since. So yesterday I was out with a friend of mine and we took a pic together..... I put the pic up as my display photo on whatsapp. She then messaged me and said nice photo. I said thanks. She said ok and take care. When I went online again I realize that she blocked me. ... Why is she mad at me when she is the one the messed. She cheated on me over 15 times but she wants to get mad??