I can't help but wonder if you aren't overhydrating. That will also dilute your blood to the point where your electrolyte levels can drop too low. 2 x 20 oz is a LOT to drink on a 20 mile ride when its only in the 70s. I would barely drink that much on a 40 mile ride if it were that cool out.

My husband gets cramps from low electrolytes. He drinks a V8 at the rest stop. We both also tried pickle juice once that was served on a group ride rest stop. Tasted better than it sounds and also high in electrolytes.