This phenomenon occurs immediately postpartum. A first-time mother may notice that her expressed breast milk is orange-pink in color. This typically comes from vascular engorgement, due to the increased blood flow to the breast. It is also related to the extremely quick development of milk-producing tissue during pregnancy. It is perfectly safe to feed the expressed milk to the baby. If the baby is at the breast, it is equally as safe to ingest the small amounts of blood being cleared. There is no medical treatment, but it will usually clear up within a week after birth. This condition should not recur.
It is also possible that blood in your breast milk is stemming from broken capillaries due to trauma to the breast, which includes rough treatment of the breast or nipple tissue from hand expression (expressing your breast milk by hand versus machine), or from incorrect use of a breast pump.
Another cause of blood in the milk, although less common, may be an intraductal papilloma. If you continue to see blood in your milk after your first week postpartum, see your doctor.