Girl treats me differently when we're alone than when we're on a group?

There's this girl from college that I kind of like. Well, she'd look at me, I'd look at her (this went on for quite a while), and I finally approached her.
Now, when we're talking one on one she's super sweet and smiles at me and looks into my eyes and tells me stuff and asks me stuff and all, but if we're with others she... doesn't seem fond of... you know, interacting with me.
More information: sometimes she looks at me from faraway, but also ignores me when we're close.
What's going on here?
Girl treats me differently when we're alone than when we're on a group?
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