How should I confront my bf about his ex girlfriend :'(?

Okay so my bf went out with this girl for 6 months which is now his ex. and he's been going out with me for 7. he said i'm his first love blah blah blah. and they went out like a yr and a half ago. well she's back in town. she's here to stay now. and she wanted to see my bf yesterday while i was over :( so we saw her and it was really awkward but we kept polite. then my bf and i went back inside his house and he apologized for the awkwardness and whatever. NOW idk if she was really his first love i'm not sure..but yesterday i was like so do you wanna hangout tomorrow and he was like no...i have plans sorta. and idk if it's with her. how do i confront him and ask him if i should be worried about him and his ex. I TRUST MY BF I JUST DO NOT TRUST HER :(