I Got Sunburnt Yesterday. I Was In The Shade Most

I got sunburnt yesterday. I was in the shade most of the time but for about 10 minutes I was in the sun directly. My left leg mainly got really sunburnt and so did my right leg. The knees hurt the most. I can't stand without excruciating pain and when I'm sitting a stinging pinch occurs in my legs. What should I do?