She probably thought you were interested in her, and she's probably checking you out because of it. Either she's interested in you, checking you out, or flattered that you showed interest. I agree with the other girls who say that (usually!) if she keeps looking at you, she likes what she sees.
If she was looking at you...then maybe she liked what she saw. I know when I see an attractive guy- - I will try to find any excuse to just glance in his direction. Try looking at her and then smiling...if she smiles back then that's an indication that she's interested