What Causes Abdominal Cramps After Drinking Magnesium Citrate?

I had a small bowel series prep done 4 days ago. I drank 10 mg of magnesium citrate to prep. Since then I have had these waves of cramping that seems to all be cramping at the same time. My abdomen , penis and what feels like my anus or colon. I have seen my gp and talked with me GI. They seems to think I have nothing to worry about. These cramps wake me up at night. Have you ever heard of this?