Sreen unlocks when receiving message

[Konrad de Ridder]

I've had my phone for a couple of months now and like it a lot, but one thing is driving me nuts!!! When I receive a sms and the phone is in my pocket, the screen somehow unlocks itself and the screen is then active as far as touches goes. When I take the phone out of my pocket later on I have found that I've made calls, had my settings changed and have even had my homecreens rearranged or deleted. Today the phone even changed the system language settings to Indonesian and I had a hell of a time getting it back to English. Does anyone know how to stop this from happening? Any help will be hugely appreciated.