How did your doctor confirm you were 6 wks pregnant if no sac was found? Just by going off your LMP? Chances are you ovulated late and are perhaps not that far along, or it may not be a viable pregnancy if your having cramping and bleeding.
I did have a miscarriage before. I got a gush of brown blood a few days after getting my positive hpt. Had one speck of pinkish red blood mixed in with normal discharge. My hCg levels were checked and found to be low, 86, when I was supposed to be 5 wks along. They were concerned about ectopic. I had several ultrasounds and none revealed a gestational sac at all, just a thick lining in my uterus. I miscarried naturally at 6wks 4 days.
I was heartbroken but I did conceive my son right after that.
I really hope your just not as far along as you thought and that everything ends up OK.