Since you've tried waiting it out and laying low, maybe it's time to take a different approach. If you're not too uncomfortable asking her to lunch or for a "Girl's Day" out together, I'd do that and see if she's receptive. Since she's a loner, she most likely won't have a conflict and could go if she wanted to. If she declines, then just be direct and ask her what's happeing - is she upset w/ you? Did something happen at school/work? Another person bother her? See what you can find out. If you're pretty sure her attitude is directed to you then you need to find out where this is coming from.
Is this girl living w/ your bf? Does she live w/ her mom or on her own? Does she have financial troubles? Is the BM in the picture and if so, is she supportive of her ex?
Good luck,
Dana