Calf cramp? Nerve pain?

I am training for my first marathon (running about 18 months, done a couple of halfs) and have developed a strange pain in my left leg and can't work out what it is. I've noiced for a while that occasionally when running I'll get these very tiny little bursts of weakness,, almost as if the leg could collapse under me, but so quick that it doesn't affect anything. After my 16-mile run last Sunday, that weakness had developed into a pain on the outside of my calf that was like a cramping, neuropathic-feeling pain. It was painful to walk on, especially going down stairs. It resolved in a couple of days, so yesterday I ran 7 miles with no problem. This morning I tried to run again but had to quit very quickly. The pain is back, though confusingly, more on the inside of my calf this time. I really couldn't run on it, and when I stopped the pain intensified before calming down. At rest, there is a dull throbbing that comes and goes. It hurts somewhat to walk on.

For background, I'm an overpronator and have had problems in the past with ITBS and posterior tibial tendonitis, sorted out with a foam roller and custom orthotics.

I am going to see a physio, but wondered if anyone has any ideas what this could be? It isn't like anything I've encountered before.

Thanks