haha ooh man I have soo been there before. I had this one friend one time in my freshman year of highschool. He was thee biggest flirt girl! And even though I knew he flirted with a lot of girl, for some reason I felt more special when he flirted with me(booy was I stupid). So he started calling me, talking to me, and texting me so I thought he really liked me since he didnt do that with other girls he flirted with. Well one day I asked him out(my frist time asking a guy out) and he had an awkward smile on his face(jsut like what you described), and said yes but then broke up with me two hours later(he didnt even break up with me in person, he told one of our friends to give me a note saying he was breaking up). I was sooo embrassed and mortified that I had asked him out and I felt so stupid, and even more stupid that my guy friend new us both so well and he had to not only find out but know that I had asked him out and he broke up with me over a note. Now I'm a senior in highschool and I have gone through 4 years of only having guy friends and a lot of relationship drama and now Ive learned so much from everything that it has become so lucid about my past and the reason why guys did certain things. Some guys have a girlfriend and are just flirts.. and it's not the first time your going to come across one of them, so before you ask a guy out, get to know him more so you won't make the mistake of finding out he "doesn't want a girlfriend" or "i have a girlfriend"(and plus it will help you get to know a guy better so that if you do want to go out with a certain guy you'll find out if he's "just not me" or if he's a "keeper"). Other guys are just players, and do not want a girlfriend and just want to date, and some just want to be in a relationship. And from experience, I know how your feeling, you feel stupid, you feel like your confidence level went down...and girl dont! in a couple years you'll look back and laugh at the stupid mistake you did and learned from... just dust it off and pertend like it never happened.. thats what i did and now that guy still treats me the same.
hope i helped!