I am coming into this convo late, but my first thought was, maybe he is 'in a relationship' with you? Or, maybe he has a stalker co-worker or friend who wants more and he is trying to keep her at bay? Who knows?
I am one who also jumps to conclusions but through this I am learning to resist the 'shoot first and ask questions later' mentality.
I would say the only assumption likely to be true at this point is that SHE is not the one, since she congratulated him on it?
