went with his ex

so me and my boyfriend are long distance rn (not to long just an hour and 30 min because of college) and last night he decided he was going to go to a haunted house. well that’s fine i told him to have fun. the whole night he wouldn’t snap me for about 2 hours and would reply with just a picture of his feet kinda made me upset but no big deal. after he got out of the haunted house i asked him if he had fun he said yeah and i asked who all went. well he named off a bunch of guys that he is friends with and two girls. i have no problem with girl friends that he has if he wants to hang out with them in a group setting but one of the girls just happens to be his ex girlfriend. that is what made me mad. when he called me later on to ask what was wrong i told him and he said it’s not a big deal. am i over reacting or do i actually have reasons to be upset?