It could be spasticity associated with MS. I get the same thing in my calf muscles. Dr prescribed Baclofen but I found that cannabis works both faster and longer.

Spasticity - Baclofen.

Cannabis...i agree with shadowbob

I have just started having the same symptom. .in my affected leg. Really strange, too. I am not moving when it happens. I can be sitting, standing or lying down. I hope it is just a temporary thing. It is just annoying now, but I don't want it to get any worse. I'm afraid to move when it kicks in in case it will grab harder.

I use cannabis mostly to help control my tremors and spasticity. It calms my shaking to minimal. I also use it for pain, sleep aid. I use different cannabis strains specially bred for certain symptom relief.