Miscalculated Pregnancy

Sarah • 23 years old married with one three year old daughter and a newborn son
My husband and I both believe that the doctor miscalculated my pregnancy. I may possibly be 2 - 3 weeks further along than they said. 
The "period" that they are going off of to calculate my pregnancy was not even an actual period. I spotted extremely light off and on for about  3-4 days. The period before that was normal. 
The growth of my baby is two weeks ahead of my actual pregnancy. 
Do you think that they may have miscalculated my pregnancy and instead of being 32 weeks I'm actually about 35 - 36 weeks?
Photo @ "32 Weeks"