Blood in the milk is one of the signs of mastitis, but there would be other signs too, as well as the blood. Is the udder ok - no heat, swelling, or the goat looking uncomfortable or off her food etc? A small amount of blood can happen in th milk, and like you say if they have big udders they can take a knock or butt from another goat.
Hopefully its a one off, just keep an eye out for any signs of mastitis, but its probably just one of those things.