Hot and cold behaviour?

I started working at a new firm a few months ago and there’s this guy that I feel an attraction towards.. We’ve exchanged a few (ok more than a few) laughs and long stares and it was nice. The problem is that the very next day, he would act cold towards me (and I get it it’s a work environnement so we have to be professional but to talk to any and everyone except me I feel that’s very strange).

And what’s even stranger is that I was talking to another coworker, we were having a conversation and out of nowhere she just drops that he’s got a girlfriend and the proceeds to stare at me for a few seconds. Honestly I didn’t look much into because it’s not like I was looking for anything to happen or like I was dropping obvious hints that I am attracted to him but I really don’t know what to make my mind up of it !
Hot and cold behaviour?
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