If you measured a week different at 28 weeks, and that one week is enough to worry you, maybe we should go over your dates.
Speaking generally, early ultrasounds (such as yours at 8 weeks) are the most reliable for determining an estimated conception date. A lot of women who complain that their ultrasound wasn't accurate for dating conception are talking about later ultrasounds. (Babies can grow at different rates.) Most of the time, in the early stages of pregnancy an ultrasound can be a helpful indicator, though not really designed for this purpose.
Diabetes is known to cause babies to grow bigger too. Also, you're further along at 28 weeks than you were at 20. By the time you are at week 40, a careful doctor asked to give an estimated conception date from an ultrasound is likely to say "plus or minus three weeks" or even more. Sometimes young women who later write alarmist posts on the Internet don't hear their doctor, they just hear a date.
If you want, write the dates you had the sex with Mr. Wrong and with your husband, the date of your first ultrasound, and the estimated due date you were given at the 8-week ultrasound and the 20-week ultrasound, and we can see if we can help you with the count and give you some reassurance.