So all you know as of right now is he posted a picture of him with some girl? I'd relax honestly. There's nothing saying it's his gf, nothing saying he is in love either, and if it was his bday he may have just been busy for the night. Thinking he's met someone and fell in love is quiet an extreme conclusion to jump to from a picture. But let's say he has met someone...still ...relax. It's natural for fuck buddy relationships to come and go. Simply bucking up and asking him if he desires to want to continue seeing you seems to be in order before conclusion jumping. That's not gonna get you anywhere but asking him will. If you're adult enough to sleep with someone you are adult enough to ask them if they want to keep doing so and he should be adult enough to tell you yes or no.