I was on mile 16 of 20 yesterday evening when all of the sudden both of my calves just locked up within about 10 seconds of each other. At times it was very painful and I almost couldn't pedal. I didn't want to stop for fear that it would get worse. I just started riding slower and over the next 2-3 miles they got better. I almost got a little panicky because it was almost dark and I only had a few minutes to get back to my car and was afraid I wouldn't be able to make it. This is about my 20th ride of this distance or longer, I work out, I eat pretty well, I'm only 26. Any ideas on why this may have happened? I'd hate for it to happen when I was 15+ miles away from home. It was pretty chilly (for me) at the end of my ride. About 45 degrees I think.