Blood behind baby at 8 week ultrasound?

Hi, so I had my 8 week ultrasound today and the baby is measuring right on time, has a strong heartbeat (171) and all looked good...until the doctor (not my doctor, the doctor at the ultrasound place) told me there is a little bit of blood behind the baby. She said it could be from implantation and the good news is it's just a little bit, and there's nothing I can do about it at this point but if I have bad cramping or heavy bleeding to call my doctor. Naturally that last part freaked me out a little bit. Anyone have this? I didn't experience it with my last two pregnancies, so I'm a little nervous. I see my regular OB tomorrow and am hoping she can reassure me...