Molar or Partial Molar Pregnancy

I am 26 years old and pregnant with my first child.  I went in for my first ultrasound on April 21st.  The Doctors thought I was going to be about 6 and a half weeks based on my last period.  However I had just stopped being on the NuvaRing for 3 years so everything was kinda off.  I was only about 3 1/2 weeks.  The Doctor was not concerned just thought the dates were off.  We went in on Monday and saw the Heartbeat of 84 BPM.  The Ultrasound tech said we look about 5 and a half weeks.  So on track according to the last ultrasound.  But the doctor says it is most likly a Molar Pregnancy.  My HCG levels on April 23rd were 19,000.  Then on May 3rd they were 49,650.  The doctor seems to be stuck now on this Molar Pregnancy.  Told me I will miscarry in a week.  But all the research I have found about Molar Pregnancies what is going on with me doesn't seem to match.  Ultrasound looked normal, heart rate is about normal for 5 1/2 weeks as well as HCG.  Why is my Doctor thinking this?  PLEASE HELP!  My husband and I are very worried.