First of all you should be sure that cramps are within your uterus because abdominal pain and uterine pain need differential diagnosis. If you are having uterine cramps then it might be due to infection or inflammation of the uterus, uterine tumors or fibroid. During initial stages of pregnancy uterine pain is possible due to increase in size of uterus. Some ladies also have cramps before and during periods due to increased levels of prostaglandins. These prostaglandins cause contraction of uterus in order to expel the contents of uterus during periods. So, it will be good to visit a doctor and get proper advice.