It sounds like the auto-display isn't working right... can you try cleaning the front of your phone, especially the little sensors at the top of the screen? Maybe there is a smudge or something that's causing it to think it's darker than it really is. Also, once you get your screen back, go into the screen brightness settings and try turning OFF the auto-display for a while and see what happens. Crank the brightness all the way up and use it like that for a bit, just for a test. If your phone goes to sleep, then wakes up and the screen is still super bright then it proves that there's nothing wrong with your display.
Try this and check back with us please...