why does my screen brightness keep going down on its own

I can hardly see the screen
It just goes down on its own.
I to have some control oer the brightness, and have got it set NOT to go to sleep on or off the battery.
It's a full whack now, and it's still not bright enogh.
Its just started doing it since i had to get my system re-set. I used to do it befoer but I managed to get it so that it didnt do it on its own.
I checked in power serttings and i defintiely do not have it set to go dim after a while.
Would love some help cos its too dark and hurting my eyes at the moment.
thanks