I go to the doctor next week but I'm just wondering can breast pain cause back pain or vise versa? I'm 21 yr old female. Painful left breast, back pain

Size. If breasts are large, they can cause a strain on the back as a result of stretching ligaments that suspend the breast tissue. Breast pain may also be related to this. In general, though, breast pain is not associated with malignancy or tumor. Other causes can be ruptured cysts, mastitis, etc.
Possible. If you have a pinched nerve in your back, you may have referred pain around to chest wall which can be misinterpreted as breast pain. Acute onset of pain in back and breast or chest wall may preceed a shingles out break.