If there is no lump just simple redness and pain consider that it is caused by folliculitis. Due to the hot weather and more sweating, the local skin of the armpit is poorly ventilated, and the humid environment plus the stimulation of sweat causes the local skin to become red, plus the friction of clothes and so on may cause local breakage causing pain. In this case, you can use iodophor for disinfection, and then apply some erythromycin ointment and other anti-inflammatory drugs for control, and pay attention to skin hygiene, keep the local dry, and reduce local friction. Female patients may also have breast hyperplasia, which is caused by the tail lobe of the breast extending into the armpit and causing pain in the affected armpit. Patients with neuritis or herpes zoster may also have pain in this area.