Blood sac under gestational sac

I am 5 weeks pregnant today. I went to the doctor today because I've had some cramping but no bleeding. They did an ultrasound and seen the gestational sac measuring exactly 5 weeks but underneath the sac was a blood sac. I know with only being 5 weeks, we wouldn't hear a heartbeat nor see anything but the sac. The doctor did a HCG draw today and I go in for another Thursday to see if the levels go up. I was wondering if anyone else has had this happen to them? And if so, what were the outcome..miscarriage or healthy pregnancy? Thank you for your help cuz I'm one worried mommy :)