Why does my ex act like this around me?

We dated for a while pretty seriously, but it ended and he chose to be just friends. so we agreed to remain friends since we were before. he has a new gf but whenever i run into him lately, he acts weird. He wasnt like this before he got his new gf, but like he has dificulty looking in my eye and acts really uncomfortable. he said he was doing great and i know he likes his new gf and he says hes over me and im like a sister to him so its none of that. he also doesnt look happy at all when i see him, but he said alls good and isnt ignoring me or anything. any idea whats going on? also i only love him like a brother now.