IPhone 4S inadvertently unlocking itself-help!

In the last couple of days, my IPhone has started inadvertently unlocking itself. I have a passcode and it is set to be input 10 times before it locks me out. However, this is clearly happening, as I have found myself (temporarily) locked out of my phone. I cannot determine whether it is the home button or the on/off/lock button which is somehow becoming compressed and thus allowing the keyboard to be activated and the numbers to be pressed. This is odd in itself, because if I press the home button, I then have to double tap to get to the point where I can input my code, so how the phone should be able to unlock itself, double tap and then insert numbers is a bit beyond me. Does anyone have any experience of this? Does anyone know of anything I can do to stop this? I have tried to manually recreate it, but it won't happen. It does happen in my handbag, but also while I am holding it in my hand too, and I don't seem to be depressing anything at that time. I have heard once that the home button can be re-calibrated. If this is something that might be useful, how do I go about it? Is there anything else I could try? Should I try cleaning the phone in case there is any dust or dirt which might be causing it? If so, what would be the best way of doing this? My IPhone is just out of warranty as I have had it 13 months. Any help would be much appreciated as this is becoming rather irritating. Thanks.