How often do you tan? Is this the first time you have tanned that much in a short period of time? If so, it may just be a sun rash. I get them sometimes if I get too much sun and I have fairly dark skin for a white guy. It could also be lotions that you are using if you do use them. Some of those are really rough on your skin, so be careful with what you put on before you tan and stick to aloe, vitamin E, or Coconut oil based lotions.

Hope this helps,
OE