I have a lump under my armpit i've always had it since puberty. it gets sore every time i pms. doctor said it was just fat, i think its something else but scared cuz i know that's a sign of breast cancer?

Other possibilities: It could also be misplaced breast tissue or other glandular in nature. I would have an ultrasound done when it is painful as a primary study. If your 1st doc won't get a 2nd opinion. Cancer would be very low on the list of possibilities,

Agree with Dr. F: Ectopic breast tissue is a good possibility as an explanation for your symptoms. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/. Get HPV vaccine. Wish you good health!