My ultrasound showed a hemorrhage behind the baby?

I had my ultrasound done 3 weeks ago, im now 13 weeks pregnant, and the ultrasound showed I have a small hemorrhage behind the baby. When I asked the doctor (not my regular doctor) she just simply said that it was blood there and didn't bother to explain anything more to me, the shock got to me and I didn't ask anything else. SOOO I want to know, Is it dangerous if its behind the baby, should I be taking extra things that I should be doing.?? Whats it caused from..... ANY help is better then what I am left with.