So I've liked this girl for a while now and recently outside of school I said hi to her when I saw her outside of school and she gave me an enthusiastic hi and smiled at me when I talked to her, the next day I saw her at the same place and she waved and said hello with a smile on her face. Since that day when I approached and started a conversation with her outside of school whenever we see each other at our lockers she'll smile at me and we'll make brief conversation and when I said hi to her (she didn't see me coming from behind her) she smiled and said hi back to me. The thing that confuses me is that when I talk to her when others are around she won't smile at me but then again I ask her questions about school usually when around others and she won't start the conversation but she is a shy girl. I've seen other signs that she likes me such as I think I saw her looking at me, she sometimes fixes her hair when we're both at our lockers together and she asked me how I was feeling after I got sick at a party. Also we'll talk on facebook chat and she'll extend the conversation with me and ask me things like "what university are you going to" or what courses are you taking and stuff like that.
The thing that confuses me is I overheard her talking to her friends and one of them said "he thought you were just being nice" and then the other says "obviously he doesn't know you very well then" and then a little later I heard someone say to her "but you don't want everyone to know about it" and she said "too late now" so to me it sounds like she either got a date with someone or has a boyfriend now. The thing is she was showing all those signs that she liked me as recently as yesterday. Is it possible she was just being friendly and wasn't really interested. Also we've never really hung out outside of school and don't talk too much.
What should I do? Did I lose my chance with her and should I just move on or should I tell her how I feel (I don't think she knows that I know that she has a date/bf and I'm pretty sure he's from a different school). Also I think she knows I like her beause I'll hear one of her friends make a comment about me as I walk by sometimes (and not good comments) yet she still is friendly to me and stuff so it seems like her friend is trying to tell her not to like me so how do I overcome this? Btw she is shy and we are both 18. If she does like me how do I get over my nerves with her and what are some things I can talk to her about? Thanks.