My iPhone screen isn't dimming when i make a phone call or take one. Why?

I've recently have noticed that my iphone 4 isn't dimming when i press my ear against it. I'm not too sure why that is. I'm not too sure if its because i just a screen protector on it or if I've messed with the settings or if its just the phone it self.

Maybe the proximity sensor of your iphone has some problems and i don't think there's a problem in settings
go to an apple service centre and they will fix it for you

The little shapes at the top, near the front facing camera, are what since if the phone is up to your ear. I guess it must me malfunctioning. I would call Apple at 1-800-6927753