The later you get an ultrasound the less accurate it is. Count the first one (at 6 weeks) as the most accurate for dating. After a while it can be two weeks off. This is because babies grow at different rates.
As early as you are its normal to have cramps.. And its normal for the baby to measure weird for awhile. You might not be exactly 12 weeks 1 day, you might be 11 weeks 3 days, or something like that. Its harder to tell when the baby is smaller.