What is this reaction I get after laying out in the sun?

For the past few summers, I've noticed that if I remain out in the sun for a few hours one day or over the course of two days, I develop a blotchy rash in the place of what should be sun burn, even when I use sun block. In the areas that I am tan, I do not have this reaction. The rashes I get on my body are mostly on my back, chest, and stomach. They are severely itchy and sting much worse than normal sunburn when they are scratched. I am 18 years old with a very fair skin tone. I do not tan very easily. I have experienced this issue for the last few years. Can someone please explain why this happens to me, what should be done to prevent it, and how to treat it when it happens?