Am I being stupid?

I'd say you're not being stupid - it seems like you are assured nothing will happen with them, but it's normal to have those feelings anyway I think. It's difficult for your bf to be really close to another girl, gay or not, and I don't think calling yourself stupid is the right thing to do. LOGICALLY you may know nothing is going on, but feelings can't be wrong or right. I and most people I know feel jealousy really easily but for women it seems we immediately jump to thinking we're crazy, when it's just normal. If you are genuinely worried he has feelings for her then try and talk to him in a non-accusatory way. I'd say if he's understanding and talks about it with you then good, but if he gets defensive and shuts you down you'd have a right to be annoyed.
Ultimately, I think anyone struggling with jealousy who is aware of their own issues and doesn't take it out on their partner deserves compassion and real advice, because from experience just being told you shouldn't feel that way does nothing except make you feel worse.

It seems pretty unreasonable to me.

no, you’re not being stupid brudda, that’s really odd. why does he have to buy her drinks??? he should be prioritising you!!! and buying you drinks over himself. mid boyfriend

doesn’t matter it was her birthday. she has more than one friend i’m sure. he doesn’t owe her drinks, he can pay for like one or two maybe but he’s doing too much. anyways none of us can know for sure because you’ve only told us a bit of info. if you’re now saying he usually prioritises you then it’s fine maybe it’s just a one time thing, relax.