21 yr old female. Achy lumpy left breast with back pain. Could this be related? Cancer? Started period last friday, off now. Cancer? Doc next thursday

Probably not cancer. At your age, the odds are tremendously in your favor. Breast cysts, hormonal swelling, musculoskeletal pain and several other possibilities are much more likely than cancer. So don't worry, but get checked as scheduled thursday.
Probably not. It's very unusual to get breast cancer at 21. However it is common to have achy breasts associated with menstrual cycle. Point out the lump you feel to the doctor you are seeing and follow his/her guidance. Avoid all caffeine and you can take anti inflammatory over the counter medication.