Terrible cramping after longer runs

My long runs have been put on hold because I have suffered terrible cramping after each one. I mean, doubled over like something inside of me is trying to tear its way out painful. It doesn't start until I am about four miles into my run, sometimes after three and a half.
I read that some runners get painful gas due to a mild lactose intolerance that doesn't present itself until they run for longer periods. I do have a mild lactose intolerance, but it was very severe when I was a child.
I was just three miles into my run yesterday when I had a painful cramp that slowed me down. I worked through it, but at the end of the fourth mile it was getting pretty rough so I called it a day.
I didn't have any dairy Saturday, just in case, but I may need to cut that stuff out way earlier than that before my long runs. I hope that's the problem, anyway.