I get this all the time, but I'm a pretty smooth guy so I've never had a problem handling this situation.
Usually what I do is, immediately upon being smiled at by this chick, I do an immediate and dramatic 180 degree turn of my neck to see if there's anyone behind me she might be smiling at(I know there's no one there, this is just for effect). Then I'll turn back around and have my jaw hanging open, at which point I'll point a finger at myself and mouth the words, "you really mean me?" at her. Regardless of whether or not she turns away, I pump my fist usually 2-3 times and nod my head repeatedly followed by a "fuck yeah." Then I'll say to the person closest to me, "did you see that shit?" You just gotta be smooth with the chicks, Frack. It's not hard. No need to be contemptuous of them for whatever their motives may be...just love them.