Am i valid for being jealous/upset?

Was at the train station today with my gf, she giggled and waved at someone i didnt see was like "do you know who it was? It was x guy".

Got quiet and upset, i knew for a fact that she and the guy used to like each other in junior high 3 years ago (we r in uni now).

She apologized and all but swears it wasnt anything like that and it was only he who liked her.

Ik shes lying since there were old posts in her old dump fb account that showed they did like each other. I wouldnt care if she waved at an old friend but an old crush?

I understand its somehow insecure on my end, guy isnt even attractive but it bothers me.