My iPhone 4 won't wake after call

Hello.

I have an iPhone 4 (gsm) that has a problem with waking after a call.

When I answer a call I get every thing as normal until the proximity sensor detects that my phone is near my face, as usual, but then after I finish my call ( not more than 5 min) the phone won't wake up.

I have a medium sized screen protecter and a case that only has a lip that is a bit taller than the screen, rulling out the bad case factor.