Ex-Boyfriend Trouble!?

Okay so my boyfriend dumped me about a month ago after 11 months of dating. When he broke up with me he didnt really give an excuse he just said that he "fell out of love" with me. The breakup was completely out of the blue to me, we were both so excited for our 1year anniversary (or so I thought). Anyways last week my friend who's still in high school had her 18th birthday party and invited me to go. My ex and I used to hang out with her when we were together so I figured I'd invite him to see if he'd wanna go (also because I just wanted to hang out with him outside of the 3 college cles we have together). He eagerly agreed when I asked him to go and a friend of mine plus him all went to the party together. As soon as he got their my friend *Becky* instantly started asking him "WHY DID YOU DUMP MY BEST FRIEND" and eventually he got in her face and almost hit her screaming at her for no reason. I had to literally drag him out the room and on the way out he punched my friends wall of her house (which is so out of character for him, he's the really quiet shy type). Outside he's on the brink of tears and he's screaming "She means nothing to me, i dont care about what Becky's saying" and I asked "then why did you react so violently when she was questioning you about dumping me?" and he didn't give a clear answer. Later he calmed down and everything was going back to normal and we were all talking and I dont recall exactly WHY it happened, but my ex looked at me (in front of everyone) and goes "Why did you invite me here tonight huh? Did you think I'd get back with you? Cuz I'm not, you'll see in the end. I DONT LIKE YOU" and I was completely embared and walked away because of his arrogance. My friends all started yelling at him "why the hell did you go to a HIGH SCHOOL party that your EX-GIRLFRIEND invited you too if you dont like her in some way?" and he goes "I came because she asked me too..." I just need someone's help and help me understand him because it just seems like his actions are contradicting his words. SOMEONE PLEASE HELP!