How far along could I be?

I got a BFP on Jan. 1st and went to the Doctor on the 4th and got it confirmed. According to them I was about 7 weeks, they went off my LMP of Nov. 7. When I went in for an ultrasound Jan 18th, there was no heartbeat. The Doc. said it was either that I wasn't as far along as I thought or that the baby wasn't developing. I have another ultrasound on Feb 17 to see if there is a heartbeat. I am just confused and scarred, I don't know what to think, I would be devastated to find out that my baby wasn't developing and that I would have to have a DNC. If they baby isn't developing would I miscarry naturally, or would I have to have a DNC. Or could I be less than I thought and barely be a couple of weeks. I didn't have a period in Dec. so when would I have conceived in I got a BFP Jan. 1st.