So I Went For My First Ob Appointment 2 Days

So I went for my first ob appointment 2 days ago and my doctor told me I was 10 weeks. I had my ultrasound today and the tech couldn't find a heartbeat and told me I looked more like I was 5-6 weeks. Is this normal?

I have very irregular periods also. I stopped my birth control in July had my period the 11th and then didn't get another period until my last one (sept. 3)
Should I be worried I had a miscarriage or did my doctor just predict my ovulation wrong?