iphone 3gs screen sensitivity REVERSED........please help ???

I have 3gs, and for the past few weeks having some wierd issues. hope i can explain it well.
I dunno what happened but my screen sensitivity has reversed, i mean to say generally when we make calls and if the phone is next to your ear the screen turns black and unresponsive to the touch of your ear and once you finish the call and remove the phone from your ear the screen activates to end the call.

in my case this screen sensitivity have reversed, the moment i take my phone next to my ear, the screen activates and thus it becomes responsive to the touch of my ear and as a result automatically the call sometimes go to speakerphone or sometimes go on hold and what not, and as soon as i remove the phone from my ear, the screen blacks out and not showing the screen end call button. i have to end the call through the top button. after a few seconds the screen appears and acts normal for applications and internet stuff. again if i make a call it starts acting in the same reversed way..............

its very very annoying and i need some expert advice. my phone is in its original condition, never unlocked, never jailbroken, with 3.1..........please please help me........hope u all understood what i was trying to say.........

Message was edited by: psax