My son gets this as well...only on one arm, and only from mid-forearm to his shoulder. It gets very warm to the touch, and nothing helps it other than coming in out of the sun and heat. After some extensive research and a chat with my pediatrician, we realized that he has Dermatographic urticaria (also known as dermographism, dermatographism or "skin writing"). He has the pressure induced version (where you can literally draw a line on the skin and it becomes a raised red welt) as well as the rarer heat/sun induced kind, hence his red rash after being outside on a hot sunny day. There is no real treatment for this condition, especially if it is mild and doesn't bother the sufferer. It isn't caused by sunscreen, but it can be irritated by chemical sunscreens. Try Neutrogena Sensitive Skin sunscreen, since that uses a physical blocker instead of a chemical one. My son is bi-racial (obviously I don't know about your child), but this can sometimes be a factor. My son's rashes usually fade within an hour, unless it is really hot, and then they tend to stick around until he has a cool bath and goes to bed. Although this may not be the case with your daughter, it could be. It can develop out of the clear blue sky, and can also be brought on by various biological/physical changes. Hope this helps!