NEVERMIND I JUST REALISED THIS WAS BECAUSE OF BURN So something I've noticed while I've been grinding is that sometimes my pokemon will suddenly jump to 0 HP for no reason at all. And I can do everything normally as if it still had its normal HP. When I finish the battle, it will display the correct HP again. But for the remainder of the battle, it will say that my pokemon has 0 HP. So far this has happened 3 times to me.