Should I be mad??

So my boyfriends birthday was on the tenth and he told me he didn't go anywhere but turns out him and two of his guy friends went to a casino for a few hours and I saw him later that day and he didn't tell me. His friend accidentally spilled the beans last night and he said he didn't want me to get mad and us fight on his birthday for going since I dislike one of the guys he went with.... But it was his birthday o.o I wouldn't have got mad but I hate things being hidden like this.... Given I've gone out and smoked with my girlfriends without him knowing but damn. Ladies please help I have been horribly pmsing and just not sure how to feel??