It's hard to say exactly what may be causing your cramping, but I'd lean toward hydration and electrolytes as a first solution.

I assume that you're in or around the Virginia/Maryland area (since you are riding in Reston)... and you have to account for the brutal weather. In this kind of southern heat and humidity, you can burn through some minerals and salts at an alarming rate. When the weather gets nasty... our heat index was 110F here yesterday... I go into full hydration safety mode for rides even as short as an hour. The Heed goes into my bottles and the Endurolyte capsules go into my pockets.

I also carefully measure how much I'm drinking along the way because I've found that most people totally lose track of what they're actually drinking. You perception of how much is enough can be way off of how much you're really drinking. Absence of thirst is a lousy indicator of hydration levels.

Also keep in mind that everyone needs different amounts of electrolytes. A bottle of Heed has the equivalent of 1.5 Endurolyte capsules in it, and on top of that I usually take 2 capsules an hour. I have a riding buddy who requires as many as 4 or 5 caps an hour to stay cramp free on those hot days.