Corn, I just had a major bout with the same problem. First my triceps siezed up... then my pecs completely cramped. My nextdoor neighbor is a former pro bike racer. He took one look at me and said, "You've got to drink Gatorade NOW. Your electrolytes are depleted." I chugged a bottle of Gatorade... and the major cramping did subside. Then I went out and bought a broad spectrum capsule (not tablet) mineral supplement... and extra caps of straight potassium. I am popping the caps three times a day for the next week.
You've depleted your electrolytes. This is just the early warning. The best thing for you to do is the same thing I am doing... replenish your body's supply of necessary minerals.