3ds screen goes black when I close it in the middle of a game

Lately there's been this issue where I'll be playing a game then I'll close 3ds and put it in my pocket. A couple of seconds later I'd open it and then there'd be nothing. The power would be on but nothing would be there. This only happened a little bit but now it happens everytime I've close it in the middle of a game. It only happens whem I close it in the middle of the game. It won't happen if I close my screen on the main menu.