Why do i get little red bumbs & a rash whenever i sweat?

Sweat Rash: This reaction is seen often and is caused by exercise-induced urticaria (hives) of a type termed cholinergic urticaria. It is caused by the act of sweating, as you have noticed, rather than heat. It is more frequent in patients with a history of other forms of hives, or of atopic eczema. Treatment can be simple at times, such as cooling the skin...or antihistamines such as cetirizine (OTC).