Why is my ex acting like a C**t?

[Eliza]

Okay, Sorry about the length but I want to give you as much information as possible.
My ex boyfriend recently ended things with me. We were fighting a lot. We had a fight in regards to his paranoia about me talking to other guys. It got to the stage where he didn't trust anything I said because of his paranoia. I thought that we had sorted things out before he left. He had told me that he wasn't going to break up with me because he loves me. Then after he leaves I get a text from him saying that he could do better without me and that he was sorry but we are over.
*I will note that he didn't delete me as a friend on Facebook and 1 of his friends added me on Facebook and snapchat.*
A few days later I get a text from his "friend" asking things like, when I was going to sleep with a mutual friend of ours. I said there were no intentions but he didn't believe me. Now he acts like he hates me. Says he has a new girlfriend not even a week of being split up.
Why is he being like this?
I forgot to mention that I am 20 and he is 21