I'm confused about this guy...

There is this guy at work, I've worked with him a couple of times and one day, about a month ago, he was my partner and he was Really flirty and kept staring at me A Lot wink at me every now and then and give me the flirty stare and smile or the flirty wink and smile he would also make comments like, "Oh You're my partner, looks like security is gonna have to keep a close eye on us!" But afterward he would say he was kidding so I just thought he is just one of those guys who acts funny and flirty, Whatever because he had a girlfriend... So about two days ago I work with him again and before we started work he seems all bummed out and not his usual happy self, so I leave him alone and let him be himself. So as the night goes on I notice he is staring at me constantly throughout the night. A few hours into hour shift I have to get something for work and he is right next to the supplies and I'm getting what I need and he says something pretty insignificant and I'm wondering maybe it was just said so he could get me to talk to him? So I look at him and start talking to him and I Do make eye contact and smile because I will admit I like the guy. So the night continues and He walks up to me and offers to be my partner and we start working then we have to take our break and we are walking and he hold the door open for me and I say thank you and he replies, in a pretty flirtatious manner "Uhh-huh!" And he then proceeds to ask me my age and my birthday. I tell him and he says, "Well I'll have to wait and see where you'll be by then." I reply laughing and tell him "You kid so much." and he asks why I say that and I say, "You told me so yourself you're a Jokester!" He then tells me, "Not This time." So we get off break I'm working and he is working somewhere else he walks up to me later on and just looks at me with... I swear it's like he has desire in his eyes or something? He tells me "Almost time to get off!" and I talk to him and smile I'm kneeling at this point so I'm looking up at him and he is looking down at me... So work is over and I'm showing my friend a short cut to her locker and the guy is right behind us taking the short cut, which he Never uses. And I tell my friend goodbye and he is right behind her and her guy and he looks at me leaving and then at them and he has this confused look on his face like, he doesn't know where to go... Can Anyone Help me and Let me know What it MEANS? He has a girlfriend so I am WAY confused about it Is this whole thing Normal? :/