Both of my legs (calves) cramped and froze while I was out running?

This has never happened before and it was really scary; yesterday I went for a run and was not even about 15 minutes when I had to stop because both of my calves started cramping and I almost couldn't walk! I tried stretching them and that didn't seem to help - the cramps slowly went away but the soreness lasted for maybe an hour and a half and it was still difficult for me to walk. I run and exercise a lot (5-6 days a week) so I'm not out of shape or not used to being active or anything. I eat a lot of fruit and vegetables and take calcium supplements and yesterday was no different. The only thing I can think of is maybe I was dehydrated, even though my water intake was really no different than other times when I've gone on long runs. Even when I woke up this morning my calves were a little sore but I can walk pretty fine now.