Red or pink milk is usually the result of a bit of blood in the milk. Sometimes the blood comes from a crack or cut or blister on the nipple (as noted above). Even more commonly, however, it comes from a tiny blood vessel breaking inside the breast. In the absence of a lesion on the nipple, a broken blood vessel is the best bet. Broken blood vessels are generally painless and usually heal by the next time you need to nurse or pump.
Bloody milk is totally safe for your baby to drink. If you want, you can leave the expressed milk in the fridge: after 24 hours or so the blood will sink to the bottom of the bottle, and you can pour off the clean milk on top.
Don't freak out if you notice blood in your baby's spit-up, as it usually comes from mom.
Sometimes over-aggressive pumping can cause broken blood vessels. You might want to try starting your pump off on the lowest setting, and gradually turning it up to full speed.