dating scan..

Vanisha.
Okay so... I had irregular periods and got pregnant within 2 months of trying. Orignally the doctors estimated my due date as 07th april 2017. So currently I would be 13 weeks pregnant.
However, when i went for my dating scan the lady said the baby is too small for 13 weeks and that I am currently 11 weeks pregnant and my due date is 19th april 2017 
I am so confused and worried cz i have a high risk pregnancy and this is my 2nd pregnancy. My son was preterm 29 weeks with CP
Would love some advice..