Just a possibility, I wear orthotics because I have a dropped arch in my left foot.
The symptoms you are describing appear to be the same. I started off by having a dull ache in the arch of my foot, which at first I was able to ignore because it wasn,nt painfull enough to stop me from running. I would just massage it and ice it and carry on.
However, a couple of months down the line, the pain got worse and spread to the whole bottom of my foot. Yes you've guessed it I had Plantar Faciitus brought on by the fibres in my foot having to work much harder and over stretch to compensate for the dropped arch.
So I would definately keep an eye on the situation I would/nt want the same thing to happen to you.
Hopefully your problems are just from the cramp.