Hey Jealousy!!? HELP!?

So saturday is our spring banquet (which is just like prom) and i am going with my boy friend. thing is his ex gf who doesnt go to our school anymore is going. Now i go to a really small school and there is only going to be about 150 kids there and i am kinda freaking out. cause we will defiantly see her. and i think she is gorgeous and i am afraid he wont like me as much when he sees her. i dont know anything about there relationship and i am scared to ask! should i just chill?! help please i am freakin out