Walked 1.0 miles again

In mid afternoon I walked 1.0 miles again, using the same route as I used on Monday. I felt fine when I finished, but I took a nap for about 1/2 hour after my walk. After I woke and was lying in bed, I got a cramp in my right calf. The cramp only lasted for a few seconds, and I got up and stretched my muscles. I should have stretched them before and right after my walk.  I've never known what causes cramps. There are a lot of ideas about cramps floating around the Internet, but nobody seems to know for sure. WebMD discusses a number of possible causes of cramps. My observation is that cramps occur in my legs due to overuse.

Two of the stretches I do are shown in the following diagram, taken from the web. I don't know if the stretches help, but they do feel good.