It happens! Can be from a few things: irritated ducts, breast trauma, or tissue changes (like PP said). Do you have any strange lumps? Chances are the blood wont be there for very long but your doctor will give you some peace of mind, and it's worthwhile to get checked out.

Side note: I had a biopsy done a little while ago for a plugged duct/strange lump and one of the effects was that there was blood in my milk. The doctor told me that most babies don't notice a few drops and it is fine for them to ingest, but if there is more than a drop or two your baby might refuse the milk because it tastes funny. Also, it's easy to see blood in breastmilk if you let the bottle sit for a bit. The blood will separate from the milk and you will see it on the bottom.