Breast pain can be caused by myriad things ranging from benign to more serious so it is important to make an appointment with your primary care doctor
or gynecologist to discuss your symptoms. Your doctor will take a full history, do a complete physical exam, and possibly order imaging such as an ultrasound
of your breasts or mammogram
before diagnosing the cause of your symptoms and prescribing appropriate treatment if needed. It would be unlikely that your breast pain is from sleeping on your back unless your breasts are very large and fall to the sides of your body causing them to sag which could cause pain. You could try sleeping with a light support bra on to see if it helps. Another common cause of breast pain is pregnancy so if there is any chance of being pregnant be sure to take a test or see your doctor. Breast pain can also be caused by fibrocystic changes, stress, cysts or very rarely by cancer. Besides making an appointment with your doctor there are some things you can try to help the pain. Try wearing a supportive bra, eliminating caffeine, applying heat to your breasts, reduce salt consumption, alleviate stress, and maintaining a healthy weight.