Internal Charley Horse?

So I keep having these massive pains (7-8 on a scale of 1-10) and I can’t figure out what hurts. It happens in either lower leg, one at a time, every week to two weeks, for a few months now. The pain’s center is little above halfway between ankle and knee, but it echoes down to the top of my foot and up to the tendons on the back of my knee. It’s like it’s between the bones, so right in the center. It feels like a standard muscle cramp, like when the big muscle on the back of the shin cramps and goes away when you push your foot to flex again, except with those, it’s clear what the problem is. With this, I can’t figure it out.
I mostly get them in the evening or at night, but that seems to be the only commonality. It could happen while sitting or lying down, and seems to be unrelated to what I’ve eaten or drunk lately. I am not dehydrated and I eat enough variety and take vitamins so potassium should be fine.
There’s nothing visible or tangible that hurts. I can massage my leg and feel nothing too-tight, and nothing hurts to push on. Sitting, standing, shifting the leg, flexing or pointing my toes, walking, or lying down make no difference to the pain. It comes on over a minute or so, hurts like hell steadily for maybe ten to fifteen minutes, then fades over 30 seconds or so.
Any ideas?

Blood clot?