SUDDEN SHIFT IN BEHAVIOR? IS HE CRUSHING BADLY? LOL?

I've been in a really happy mood these past 2 days. I work with this one guy and he's been looking at me a lot lately, but I avoid much of the eye contact as to not lead him on or anything. I have a flirtiness/friendliness about me but I don't do it for attention. I just wear my emotions on my sleeves and I like most people I meet. So, he usually initiates a greeting or sings around me or tries talking to me loudly/freely these past few days, especially. He teases me too and I have some pretty funny reactions that he enjoys, so after some teasing and small talk all of a sudden he became really quiet around me (no singing, etc) but very attentive to everything I said and did. He listened to one my conversations and asked, "why are you working here?" because of the way I talked about nutrition suggesting I'm smart--his exact words on the topic "you sound like a nutritionist!. He also avoided most of eye contact and had a hard time looking at me w/o like smiling big! lol Also, when he did talk to me in private he would smile big and like there were sparkles in his eyes, but when I walked out he kept his head in his paperwork and just gestured a "peace" sign w/o looking up or saying anything.