I had an irritable uterus which felt like cramping. Imagine, a small uterus growing a human getting stretched and kicked - of course it would get irritable! ;)
any cramping, especially if it is rythmic, or more than 6 contractions per hour, need to be discussed with your doctor.
If you are ever concerned, you can call and talk to the nurse at your doctor's office. They are .G.R.E.A.T at putting your mind at ease.