It used to be when people did this to me I thought they were accidentally looking at me and then embarrassed when I saw them, and since I'm apparently evil, I would point and laugh at them because 1) I didn't understand that embarrassment is a really bad feeling and 2) I think it's funny how people look when they get embarrassed. Then I found out that this is actually often a form of flirting, and I feel kind of...strange.
I hadn't originally thought that people would ever flirt at me, so it had never crossed my mind. Maybe the girls were flirting with you.
I feel like maybe it was mean of me to laugh at the people, but I really do think it's funny! I laugh just thinking about it.