I too suffered from similar cramps when I returned to riding four years ago. I an totally empathize with you for I feel your pain.

By chance are you taking birth control pills? Some BC will actually pull potassium from your system. When my doctor and I ruled out low potassium levels we came to the conclusion that my cramps were simply due to over excursion and she suggested I cut back on my mileage some. When the winter months arrived and I quit riding the cramps disappeared. When the new season started I started out with moderate mileage and simply increased my mileage by 10% each week. That seemed to do the trick.