HI, I'm 16 and a girl.
And I recently broke up with my boyfriend,
No matter why you broke up, the feelings you had for each other will always be there, in my opinion, even if you have moved on, and are "over" someone, you still remember what it was like before, so maybe when she sees you, its just some of those left over feelings that she has, I have certantly done this before.
It could also mean, that she still does have feelings for you, is not over you and wants you back, I can't really say without knowing the situation. But who broke up with whom? if you did the breaking up, its very possible she's not over you, but even if she did it, its still very possible she still does like you, I have been in that situation.
If you reallly want to know, then maybe you aren't over her. Try smiling at her the next time you see her looking at you, and maybe wave or at least acknowledge that you see her and that she means SOMETHING to you.