Lower calf pain was the first recognisable symptom I had 20 years ago (alongside migraine) and has persisted ever since. For me its a throbbing, diffuse pain that is exacerbated by tiredness. Painkillers don't do much for it, but even small amounts of sleep will reduce it immediately. It is usually a good indication of the arrival & severity of PEM for me.
Luckily for me I've found that taking a good vitamin and mineral supplement has reduced the frequency/severity of it, along with frequent uncontrollable muscle twitching that I also experienced regularly. Potassium/magnesium insufficiency has been mentioned?