This has been happening to me, too. I think that it occurs because you are good-looking, and guys don't want to have to face any sort of rejection from an attractive woman (even if it might be something as small as the girl not returning glances). At least, this is the conclusion I have come to. Plus, to put it another way, I look/glance at all sorts of levels of attractive guys, even some not so handsome ones, simply because I like to look around and see my surroundings. Therefore, if it seems as though guys are avoiding looking at you, it is probably because they are intimidated, unless you are extremelyyyyy bad looking, which does not appear to be the case. So, although it may seem frustrating, it actually might be a good thing.
So guys looked at you before but not now? Is there a difference in your appearance?