Should have Known...

So I've posted about this guy that I had been seeing, I've been seeing him since May (not official) but it was going in that direction from we had talked about... I've had a GUT feeling that I wasn't the only one in the picture but had no proof. Recently on social media he posted "in a relationship" when I asked him about it he said he was just joking to see how I reacted to it. This one girl had always posted things to him so I did a "Fuck it" and messaged her & asked if they were together. Yes indeed they were. OUCH! He's 28 I'm 24. He flipped it on me saying I was acting childish & so on. I was hurt. We work together & the fact that he made me look so stupid. Always go with your gut feeling & Never date anyone from work. I had to vent. Thanks guys! Have a good night.