Not a good workout today.

I run on the treadmill 30 minutes a day, for some reason today about 10 minutes in, my right calf completely cramped up, hard! I hopped off, stretched it out and got back on, the next 5 minutes was far from enjoyable. So I cut my losses and got off. Finished the rest of my workout like arms and abs.
But do you think it is a sign of dehydration? I drink a lot of water, but we've been spending a lot of time outside the last week or so, with the heat, maybe my water intake isn't where it should be???...