Why's my ex boyfriend acting this way we ain't been together in 3 years?

I ran into my ex boyfriend Jeremy yesterday, at golden corral and when he first seen me he looked like he wanted to give me a hug but acted like i didn't know him,and he was with his girlfriend and the whole time they were there..i inore him and he acted like it hurt him in a lot of ways like for one i walked ped his table and he looked at me n a sad way and i almost cryed i rolled my eyes while i was looking straght ,i never looked him in the eyes next thang you know.. he's telling his girlfriend something and there laughing at me i just acted as if they werent there he kept smiling when ever his girlfriend went to the bathroom or if he was alone he asif he was so happy with her..but the whole time they just stared at me...then for a while he started acting like he was inoring me but his girl was watching.but when his girl looked at me i called her a b***h then when ever i smiled they would stare at me.they were finally leaving and he kept his head down like he was sad as he left