Girls how should a guy know if a girl is just really shy around him or not interested?

There's this girl that might be into me because she is a little shy around me but she also makes me think she's uninterested too. She's a friend of mine from college and school had just started up again. We've been back for a week and I've only seen her once. Last year she was a little shy around me but I thought she might be into me by the way she was really smily towards, stared at me a lot, and I even had people telling me I could get at her. But I didn't because I was nervous last year. But in the beginning of the summer I started to text her a little bit and she seemed happy that I would text her. Sometimes I would say nice things to her because she is a very nice girl and deserves to be treated nicely. But After awhile she stopped replying to me so I stopped texting her until we got back to school. I've seen her once seen we've been back and when I did she came up to me to give me a hug and we talked a little after that. But as the night went on she got quieter around me everytime I talked to her. But when I started talking to other people that's when she started staring at me a lot. So the next day I texted her saying it was good to see her and that I thought she looked pretty and gorgeous last night. We talked for a little bit but she doesn't have the same interest when I text her anymore. The last time I texted her she was real short with me and didn't seem to care much. When I first started to text her in the summer she always seemed happier when I text her. Now I just feel like I'm bothering her. I can't figure out if I'm bothering her and she's not interested or maybe that she is just really shy around me. I have no idea what she thinks of me and I have no idea what to do :/ Should I keep trying with this girl or should I just give up? She one of the nicest and prettiest girls I've met and I want to keep trying with her but I just don't know if I should. She's not out of my league or anything either. So girls if you could me advice please do
Girls how should a guy know if a girl is just really shy around him or not interested?
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