Sometimes when I run, one or both of my calf get painfully tight and can remain sore for days. Vigorous deep massage with a machine (painful itself) and stretching help some but not much. What is going on at the cellular level that causes this? Why might it happen to one leg and not the other? Do the muscle fibers actually become inflexible? How/why? Are there too many microscopic tears happening? FWIW, I’ve gone from zero to 4-5 miles per day, 5-6x week over the past three months and occasionally get horrible cramps in my calves at night. But those seem to have decreased substantially since I began running regularly.
I had a similar, if not the same problem last year. My physiotherapist explained it to me but I can’t remember now. The exercise he gave me was to stand on my tippy-toes (balanced against a counter or something) then gradually be able to stand tippy-toed on just the bad leg. It will strengthen the calf muscles. Are you rolling out with a foam roller after each run? That should help too. Good luck!
Stretch before exercise, hydrate and rehydrate properly and replace electolytes (milk is easiest).
What you’re describing falls under the general description of muscle cramps. It’s muscle fibers that are in a state of continuous contraction.
There can be many causes and I think WebMD does a pretty good job of explaining what’s known.
Or, according to some, don’t stretch before exercise.
Yeah - warm-up before exercise, stretch after. Warming-up can include dynamic stretching once you get going.
I use dynamic stretching for a warm up. It is really important, however, to static stretch after exercising. It will help build muscle too! (Possibly) 
WebMD gives possible causes and treatments options but doesn’t address what is actually happening chemically or at the cellular level. It seems nobody really knows what cause cramps just some things that might contribute. Unless I’m mistaken, muscle action is caused by electrical and/or chemical signals. How are these different from normal when a muscle cramps?
I ran today and felt one calf starting to cramp up after a few miles. I stopped and stretched it for a minute or less and went back to running. On a whim, I lengthened my stride a bit and by the end of the next 1 1/2 miles the cramp was completely gone.
Unless lack of flexibility is an issue that is holding you back in your life somehow, what exactly is stretching supposed to accomplish? Fight my ignorance please.
I stretched in the Army and it never did anything except kill 10 minutes I would have otherwise spent exerting myself. I haven’t stretched regularly since (except for semi-consciously when my muscles get sore or tight) and it doesn’t seem to cause any problems. What am I missing?
I don’t think “tightness” is the same as “cramps”.
What causes just tightness after exercise?
WebMD does:
When a cramp occurs, the muscle is in a constant state of contraction. In my experience, a calve cramp is caused by overexertion of the calf muscle. These sometimes happen to me as I lie in bed with toes up (due to previous exercise). I get up, let the toes dangle down, take the weight off the leg, and massage the calf.