How To Treat Excruciating Pain In The Knees Due To A Burn?

I got burned very badly at the river yesterday, even with a few applications of sunscreen. Today I am burning very badly, but worst of all when I try to stand excrutiating pain shoots through my knees and I have to flop back down. I can not express how bad this pain is. What is going on and how do I relieve this pain?