I have sever left breast pain with a large vain that goes through to my nipple what could it be?

Get medical check: Pain in the breast is usually due to a benign cause(breast cancer is usually painless). There can be infection or inflammation (? mastitis or Fibrocystic condition). If the pain persists for more than one week, you should get a physician to examine you. If you are over 40 years old, you may benefit from a mammogram(X-rays of the breasts).