I have a lump under my right nipple. it's tender and been there for about 4 months. no other symptoms. i am 34 and male. what could it be?

Gynecomastia perhaps: -breast development in males. Reflecting imbalance in ratio of estrogen to testosterone. Usually benign. See md or general/breast surgeon. Medications, pot, dietary /hormonal supplements, liver disease and obesity may contribute. Surgery is option if bothersome. Other tumors possible. Cancer rare, check family history