Why do women always try to check and see if men are looking at them?

As a woman I am very guilty of this but I tend to feel like there are eyes on me even when there isn't sometimes. There was one point when I was a little heavier and did not feel very attractive from behind and I would feel self conscious about they way my back side looked or jiggled...I was constantly looking behind me to make sure that no one was making a sour face behind my back.