There's a few possibilities. Does it look like a boil/large pimple - if it does, then you need this treated by a doctor. It could also be an enlarged lymph node or sweat gland. The 3rd thing is what the other posters said - the breast tissue and breast glands to go into the armpit region....so monthly tenderness can cause discomfort. If its a sore sweat gland/lymph node, use a clean razor when shaving your armpits. If you recently changed your antiperspirant - sometimes this can be the culprit....you might even need to stop wearing antiperspirant for a few days . Some people do get recurrent infected pores in this region. After shaving you can put some hydrogen peroxide on your armpit, then use antiperspirant.