Why is my ex so nervous around me?

My ex and I were in same friend group, we dated for about 3 years. He broke up with me because he was going through some stuff and was "not ready for a relationship" but told me constantly how much he loved me and about how he was "not sure about the choice" he was making by ending it with me.

I was understanding and did not say anything and let him do his thing and accepted his request to still remain friends. During our being friend period he would talk to me and treat me like we were still dating and it made things confusing and difficult for me, when I asked him he said he really cares about me and would hate to lose me. About a week or two later, I found out he was dating a girl from work which was hurtful because I fel played. He then messaged me that he cannot be friends anymore (2 days after their 1st date) because she is uncomfortable with me and basically made it seem like he was only being friends with me because he "felt bad for me". I was mad and told him he is terrible at communication and left him on read and started no contact.

Months went by and ran into my ex at a party, I had found out from mutual friends that he and the girl had lasted no more than 2 weeks and he had been acting incredibly strange. I kind of took all of his actions as just a sign that he is going through something and did not want to add more to his plate, so when I saw him in person I apologized for the comment about communication I made and extended an olive branch to clear any sort of drama.

He was glad and because of that now always attends social gatherings and things I am at, but looks incredibly nervous around me, sad and is unable to make eye contact. On top of that does not say anything to me at all and quietly sits on his phone every time I am at the same place. All of my friends also find it weird. I am friendly and will say hi and check, but I'm confused at why he is so nervous? Also if he thinks "we are cool", why is he weirder?
Why is my ex so nervous around me?
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