1st time Mommy to be and confused about Due Date.!

I got a positive test on July 4th. pregnancy was confirm with two trips for blood work (numbers was low) so I went in for a early scan at about 6 weeks or so (heartbeat was detected) and I was told my due date is March 19 based off measurements. I was on Clomid and was temping/opk so I know exactly when I ovulated which was June 21. It's just not adding up by a week. I know a week off is not a big deal I'm just new to this all and wasn't sure why baby wasn't measuring up to the size based on when I ovulated . Could I not ovulated when I thought .? Or could I be a week further then my doctor actually think .?