Blood in uterus 6 week scan


I just left my first ultrasound. I'm 6 weeks. Baby is doing really good we were able to see the heartbeat right away it was incredible but when the doctor came in he said that he could see some blood in the ultrasound but not to worry and he wanted me to come back in 3 weeks to see if it was gone. I was freaking out and I didn't ask anymore questions. Now I'm so scared ...has anyone gone through this ??