What cause and cure for tense or cramped stomach muscles? I recently restarted cycling to work. I did four round trips. After riding back from the fourth trip, my stomach muscles started to tense and cramp. 

I heard that stretching before and after a ride, as well as remaining hydrated, could prevent this. Is there anything else I could do to prevent cramps? I intend on riding four days next week too because of the lively feeling I gain from cycling.

EDIT

There is nothing abnormal about my medical circumstances.

I ride 9mph on average in one direction (using cycle paths rather than roads) and slightly less coming back.

A one-way trip is under four miles.
Of course, as with any exercise, stretching is worthwhile. Mostly a few slow warm up laps around a short course, or taking it a bit easy the first few trips out, as you get used to riding a bike again. Mostly, try riding 2 or 3 days a week, with a rest day between, until you get fit enough to ride comfortably.