Why is my ex doing this?

My ex and I are cordial to eachother. He has a gf who doesn't know I exist. She's out of the country. We share mutual friends-therefore we end up at the same place at times. We ended up riding together to a party this wkend with another couple we know-one of which is my bff. My ex has been acting like a lately actually-he 's usually pretty extroverted and friendly-as of late he's quiet and isn't drinking at all when we all go out. I'm seeing someone new but he moved on before I did. Everytime someone asks, mentions, something about me at all-he always chimes in with deatils and some of it is eerie stuff he remembers about me, my fam, life, etc.... This girl he's with is so not right for him-I genuinely want him to be happy. I'm over it and done-time to move on. So why is he acting like an *** one min and turning around and chiming in about my life the next?