Well, there are two possibilities. One is that she is checking you out. Remember that in most cases, if a woman is not interested in you at all, she will not even "see" you. So the fact that she is looking at you could be a sign that she is curious about you and checking you out.

Of course, there's also the possibility that she's looking at you because she thinks you look or act funny in a bad way. This is quite rare because women often IGNORE the guy they find disgusting to avoid any contact. However, when a group of girls gather together, they may look around for guys to make fun of because it is "safe" to do so.