I'm a 16year old. i have a very sore lump under the skin of my right breast. it is located beneath the nipple. i'm terrified, what could this be?

See a doctor: In your age group, the likelihood of it being a serious condition is low. There is the possibility of a clogged duct, or gland. Try applying a warm wash cloth/compress. If does not improve or if redness spreads then will need to see primary doctor or gynecologist

Breast pain: You may have an infected hair follicle or a blocked milk duct if it in the skin surrounding the nipple. Breast cancer in your age is very rare and malignant breast masses are generally not painful. You can apply warm compresses and an antibiotic ointment 2-3 times daily. If it doesn't go away in a few days, see your doctor.

Anything-go to M.D: This could be anything from an infection, to a blocked duct to a cyst to a tumor. Does it change with your menstrual cycle? The only way to know what it is, is to check it out, Go to your doctor for a breast exam. Generally tumors do not hurt and are unusual in your age group, but it is not impossible. Better to check it our instead of worrying. You may need a biopsy, if the lump doesn't go away.