The most common cause of a small pimple under the armpit is skin allergy. As the skin under the armpit is moist, it is easy to cause symptoms of allergy. After the allergy, the surrounding skin will have a rash, there will be small grains under the armpit situation, anti-allergy treatment can be carried out, but the drug needs to be used under the guidance of a doctor, do not blindly treat. The second common cause is folliculitis, the so-called folliculitis, is our normal hair follicles are infected by the surrounding tissue, causing purulent or non-purulent infection, thus raised skin, forming a small granule-like state.