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.
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.