First dr. Appointment today after my BFP

So I went to the OBGYN today as I got my positive test some time last week. my last period was October 5 then I had light pink bleeding on October 29. As excited as I was to have my first Dr. Appointment I was kinds disappointed and in some what denial. So the Dr. took an ultrasound and it shows am only about a month pregnant and there is no heartbeat. So he schedule me to return in three weeks for another ultrasound to see whether or not the baby is growing. So my question here is how is it possible that I haven't had my period in two months but am only one month along. Has anyone here experience anything like this before and would it be to soon at 1 month to get a heartbeat as the Dr. said there is no heartbeat as yet. Am a little bit concerned.