off birth control and pregnant

I decided to get off birth control in November and only had one period which started on November 30th and ended on December 5th. Got pregnancy confirmation on the 28th of December. Just had first prenatal visit and only measured at 5 wks 6 days(6 wks 4 days as of today). Is it possible that my birth control threw off my ovulation date or should I be worried? Doctor put me on progesterone and said everything looks fine but I'm still worried. Calculations seem off to me. (Have second ultrasound feb. 17th-sorry first time mom so I'm nervous)