You are probably thinking you have cancer and are worried sick. That is unlikely at your age.
A young woman your age may experience a number of changes in her breasts during puberty and adolescence as she becomes an adult.
Cyclical breast pain, the most common type of breast pain, is associated with the menstrual cycle and is nearly always hormonal.
The pain which is almost never associated with breast cancer may be felt in only one breast or may be felt as a radiating sensation in the under-arm region. The pain may also be a result of hormones received from contraceptive methods (i.e. birth control pills).
Try to avoid caffeine (coffee,chocolate colas etc..) because it can increase your breast pain.
Some of the breast conditions young women may experience include Cysts and Fibroadenomas both benign lumps that could cause the pain you are describing.
It's very wise of you that you made an appointment to see your doctor.I am sure it's nothing to be worried about,but any lump in the breast no matter what the age of the person is,should be examined by a physician ,just to be on the safe side.
Take care and best wishes...