Fetal Pole, but no heartbeat detected.

I got my BFP last Friday. I had some cramping and since I have had 2 miscarriages previously, I decided to go to the ER. The ultrasound showed a sac and yolk with a fetal pole but no heartbeat. The ER doctor said it could be too soon to detect or it's sign of a miscarriage. Has anyone else had this happen with a positive ending? My last period was 12/1/17, but lately my cycles have been 40 and 47 days. I am set to recheck HCG on Monday to see if levels are rising.