so yesterday i learned of this art and photography exhibition at the museum next week.

i immediately thought of going with my boyfriend since we're both into art and photos and haven't actually gone out proper for the last few weeks cos of exams.

today i asked him and turns out he's going with a couple of his buddies...and he knew about it much earlier but didn't even tell me.

feels like shit cos i thought of him immediately but he didn't

Well, just let him know that you would have liked to go with him, and would appreciate that next time he thinks about it by himself.

But there's no point in wasting time and energy on that subject, unless it happens again

As mentioned, could be a problem if it happens again.
It could signify something negative, but it's also a poor expectation that he'd stop doing these things with his friends just because you came along.
(Wait.. he goes to museums with his buddies... is your boyfriend gay?)