Every month, i get pus filled knots under my arm. i have gone to the doctor and was given antibiotics. the lumps go away, but they keep returning.

Dermatology: It could be hidradenitis suppurativa, a disease which effects hair follicles primarily in the armpits and groin. Treatment with antibiotics sometimes isn't effective enough and the condition recurs. You should be evaluated by a dermatologist to investigate all of the treatment options and to work-up any underlying inflammatory disease.