Hi,
I've been complaining of sore feet lately too (I'm 31 weeks). Not sure where you're located, but the weather has cooled in Melb/Vic the last couple of days and my feet are not so puffy or sore. I haven't had any cramping like you've had, doesn't sound like much fun. Are you raising your feet up on the couch when possible?
I wouldn't be too concerned about it (unless it keeps cramping and you can't get any relief), as it just sounds like 3rd trimester in summer. I had massive kankles and sausage toes with my first summer pregnancy (a March baby) and then even bigger feet and kankles after the emerg c/s. They were huge. I distinctly remember a couple of weeks after birth, marvelling at my 'dainty little feet' again, and seeing the bones and normal looking toes again.
I think you can soak your feet in Epsom salts to see if this helps reduce the cramping. As I type this last sentence, I'm wondering why on earth I haven't done this myself, yet?
Hope your feet calm down for you soon