The first patch of about 10 blisters lasted for a week. The second is still present, but on the other side of my back. I live in south Texas and spend a great deal of time in my car. Rashes can be caused by a variety of conditions. It would be a good idea to see your primary doctor or a dermatologist for an accurate diagnosis and treatment plan.
Heat rash does tend to occur in hot weather.
It shows up as a rash of pinhead-sized bumps surrounded by red skin. The rash usually appears on the neck, upper chest, groin and armpits. It is caused when the sweat does not flow freely but builds up under the surface of the skin. The best treatment is to keep skin cool and dry and to avoid situations that cause perspiration. After exercising, it is helpful to remove wet clothing as soon as possible.