The causes of hot underarm pain usually include folliculitis, sweat gland and acute lymphadenitis. 1. Folliculitis: the hair follicles in the armpit are widely distributed, plus the sweat glands in the armpit will secrete thick secretions when it is hot, when the hair follicles are blocked and folliculitis is triggered, the pain in the armpit will be produced under the stimulation of inflammatory factors. 2. Sweat gland: when it is hot, the sweat gland in the armpit will secrete a lot of sweat, if you don’t pay attention to hygiene, it is easy to breed bacteria, which will lead to sweat gland infection and cause sweat gland, if the condition is not controlled in time, it will affect the excretion of sweat, and a certain pus cavity may form, which will lead to underarm pain. 3. Acute lymphadenitis: the armpit has the largest group of lymph nodes in the upper limbs of the human body, if the armpit lymph nodes are infected by hemolytic streptococcus or staphylococcus aureus and other pathogens when it is hot, causing acute lymphadenitis, resulting in lymphatic reflux obstruction, lymph node vascular dilation, lymph node enlargement, local pain and other symptoms. If you have underarm pain in hot weather, you should go to the regular hospital in time to find out the cause of the disease and choose a suitable method of treatment under the guidance of professional doctors.