Why is my ex being so rude to me?

We broke up, like more then a week ago because he still had feelings for his ex, and he flirted with my friend. Also it just didn't seem right. So today at school, it was our last day & he hung out with my friend the whole time & I found out that they like each other even after he swore he never did... & he also gave me dirty looks all day & would be staring at me. I found it rather rude, he also called me a coward for not breaking up with him in person--but he broke up with me over text? Idk he has like a bunch of mood swings, just a couple days ago he made a small convo with me on msn asking what I was up too, and he told me he was just making plans with a few friends for July 9th. But yea.. ever since we broke up I been as happy as ever, and I think he notices that & thats what got him mad. But it was a crappy realtionship anyway... If your happier now then you were with him then that tell's you something doesen't it.. but I don't get why he is being so rude.