Why he has not text-ed or email back?

Ok! I work with this guy that I have a crush on. He knows I like him, and we flirt all the time. We have kiss a couple of times and we actually when out one night to a bar only us two. What I don't understand is why sometimes he acts like he doesn't care? Some days he will be mega nice and other days he wont. We were fine on the weekend, I text-ed and email him on Tuesday to see how he was doing and never got a respond back. I got to work on thursday and saw him and he acted a little different. I told him I TEX-TED HIM on Tuesday and he acted like he did not know and smile at me. What is up with that?????? Please help... : (