Why Am I Spotting Brown Blood During Pregnancy?

I am approximately 6 1/2 weeks pregnant. I have been spotting reddish brown blood every day for the last two weeks. I have never had any of the normal pregnany symptoms (sore breasts, nausea, etc.). My last internal ultra sound was last friday and the doctor could see the yolk sac and said so far it looked like a normal pregnancy, despite the bleeding. Should I be concerned? Also, I am tired a lot more than usual and I woke up this morning with a sore throat and I have been sweating on and off today...