It would be helpful if you provided more information, describing the rash that you have developed, and the circumstances under which it developed. Many people wrongly diagnose other skin conditions as heat rashes. This is of course very common in cases of self diagnosis, so unless you are sure that you have a heat rash, you should let your doctor have a look at it on your next visit.
Heat rash is also known as prickly heat, and its medical name is milaria. It is very similar to acne, in that it is caused by the same mechanism involving the blocking of sweat pores, and also involves and infection by the same bacteria. Our skin is covered with tiny pores, and each of these pores is connected to sweat glands. Sometimes, these pores get blocked by dead skin cells, dirt, and bacteria that naturally occur on the skin. This tends to happen most often during the summer, if the weather is particularly hot and humid. As a result, the sweat glands are unable to release sweat, and with the added presence of bacteria, the skin begins to itch and prick, and tiny blisters appear. Prickly heat or heat rash usually occurs on areas such as the neck, back, armpits, under the breasts, and in the groin.
Fighting the heat is an important part of tackling heat rash - wear light, cool, loose fitting clothes, made of natural fibers, and try to stay in cool environments as far as possible. You should avoid exerting yourself unnecessarily, and also bathe several times a day, preferably using a cold shower. Antibacterial soaps and antibacterial body powders are also available, and are quite helpful in minimizing the severity of the rash as well as in reducing the duration of the outbreaks. If the heat rash bothers you in spite of these measures, you should consult your doctor.
answered by M W