This is quite common for first time mothers or if it has been awhile since your last pregnancy. Many times it is caused by very minor damaged to the inner capallaries of your breast tissue due to the expansion internally. It can be very scary but most of the time it is harmless and will go away on its own! There is actually a name for it which let's you know it is pretty common, it's called "rusty pipe syndrome" because often the blood is a rusty color..
I have been a doula for almost seven years so feel free to contact me with any other questions! Best of luck on your pregnancy! Most of all don't worry, but do call your doctor or midwife if you are still scared. Beware of scary images online. They do more harm then good!