The last time I had unprotected sex was Aug 1, 2012, my period after that was 4 days instead of 7. When I went to the doctor yesterday they told me I was 5 weeks... Is that correct?

What was the first day of your last period?

Aug 8 and that was the 4 day period

So where you having "protected sex" since no protection is 100 percent.

There is no exact conception dates, they merely use last periodi as a guide line. So if you where having protected sex after the 1st, it is possible that was when pregnancy happened