Im a very bubbly, and my personality is flirty as it is, but a newly employed coworker at my job just has me at a loss for words.  I always joke with all my coworkers but we both catch each other flirting with the other and I feel so wrong like as if I'm cheating. He has a girlfriend too but he complains about her a lot and so he never hesitates to flirt, but I don't want to catch myself getting into a bad situation. 

I thought it was meaningless but one night he went out drinking and texted me twice during the night asking me if I was up and telling me to wake up at like 3:30am so obviously he's thought about me. I just don't get it.