A little swelling under the glans and a little pain when it hardens may be caused by factors such as penile sclerosis, trauma, and glansitis. 1. penile sclerosis: may be related to chronic injury, heredity and other factors, can make the glans localized protruding plaque, mistaken for a lump, and in the penile erection will appear obvious pain, and in serious cases will lead to sexual dysfunction as well as urinary disorders. 2. Trauma: such as surgical injury or external injury, resulting in localized damage to the glans, which may cause symptoms of swelling and pain, and the symptoms of pain will be aggravated during erection. 3. glansitis: may be related to poor hygiene or unclean sex, by inflammatory stimulation, resulting in damage to the glans will cause swelling, in the erection, easy to cause painful symptoms, may also be accompanied by itching, redness and other symptoms. The lower part of the glans is a little swollen hard and a little painful, it is recommended to consult a doctor in time for examination and treatment, to avoid continued aggravation of the lesion.