That must have been very scary for you Luckily blood in breastmilk is very common and is not a sign of a problem - it is also fine for your baby if they drink some blood.

There are three main causes (other than bleeding nipples) - rusty pipe syndrome, which is usually common in the early weeks with a new baby and is related to the developing bresat tissue, broken capillarires usually from a bump to your breast or other rough handling, or the one that sounds like what might be going on with you is one or more benign tumour in the milk ducts (I know that sounds bad with the word tumour, but these are benign so no need to worry). It will stop by itself in time and there is no need for treatment (but consulting your GP was a good idea to put your mind at rest). These tumours can be painful but not necessarily and you won't be able to feel any lumps from them. Notice that mastitis is not a cause of blood in breastmilk!

I hope it stops soon for you hun, I know seeing blood is never nice!