You could be allergic to some unknown substance or this could be sweat rash or some infection.
Do you have any other associated symptoms?
What work do you do? Do you handle any chemicals or soaps or detergents etc?
You could wash the area several times daily and apply calamine lotion at the sites. Take oral antihistamine medications like loratadine or cetrizine for your symptoms.
Try to find out the cause of your symptoms or any factors that aggravate the rash. Consult a skin specialist for a clinical examination.
Let us know about how you are doing and what your doctor advises.