Worried 😕 Rush Doctors Appointment

QuickSilver
I had my first scan yesterday at what my doctor thought should be 8 weeks. I knew I wasn't that far along, I figured only 7. Well the ultrasound showed I am only 6w3d which didn't really distress me. The radiology place booked me in for a follow up scan on Dec 1 to look again. 
The second week of November I had to get my hCG checked twice because they felt it was low when I had it checked initially on the 4th. They don't release numbers over the phone but said they would call if anything was wrong. I didn't hear anything so I figured it was fine. 
Anyway my doctors office just called and want me to come in for an appointment in half an hour. I already had a scheduled appointment for Thursday so now I am a bit worried that they are rushing me in.  Needing positive thoughts and reassurance. Hopefully everything is fine and the doctor just calculated my conception date wrong. 
*UPDATE*
So the doctor isn't super concerned. She said most likely I am just way earlier along than they thought since my cycle was not regular. My hCG they took was really good. The first number was 2000 and the second was 5000 (I would have been between 3 and 4 weeks then). She is sending me for hCG again, just to be sure, but she says the ultrasound I have booked for the 1st of December should show us more accurately where exactly I am in the pregnancy. I'm relieved for sure.