BB Bold :: 9900 Screen Won't Turn Off When Phone Is Locked?

I have had my Bold 9900 for less than 3 months and in the last week I have experienced the following issue: The screen stays lit up constantly, which is making the battery run out very quickly. When I press the lock button the screen turns off for about 5 seconds, then lights up again, and stays lit. I can't see anything in the Options menu that will fix it. As I say, the problem has only been occurring in the last week.