Sounds like the same problem i have currently. I've done some strength training for 3 months and completed 3 PBs from 10k to marathon, then suddenly calf tightened too much. After resting (eventually!!), it was OK for 2 weeks, including a 19 miler. Had MOT at physio and he advised stretching (as I was) and then again it was very tight today in a 10 mile race. I held back (as I didn't fancy walking 5 miles back to the start in the rain) and at 7 miles I thought it was going to go completely, so eased right back for a few minutes. It then cleared and I finished very strongly, so I don't know what the solution is. Rest, stretching and massage are the obvious answers, but from my experience, that is not enough. If anyone has the remedy, I'm all ears.