You could just be seeing things. A lot of people smile at someone else as a way to kind of say "hello." I can't count the number of times another woman would smile at me while I was walking down the halls of my college.. but that's all it was... a kind of greeting.
I think that because you want her to be blushing, you kind of see things. But, give it a shot. Next time you notice a girl smiling at you and you think it's because she has interest, act on it. You can't get over your loneliness by not acting on this stuff.
I don't chase, I replace.