Why is my sunburn so painfully itchy?

On Thursday I was in the sun for a bit too long and the UV index was 9. I usually don't burn so I used an SPF of 4 just to be safe, it turned out that it wasn't high enough or I didn't put enough on because I got a really nasty burn on the backs of my legs. I've stayed out of the sun and used plenty of aloe and taken tylenol to reduce the pain. Everything was fine until today, after I took a shower my sunburn began to itch like crazy along with pain all over my burn. If I scratch the burn it hurts and the itch won't go away and even if I don't scratch it I'm writhing in pain. Please help!