This was happening to me at work often enough that I actually had to invest time in figuring out why. It's really, really embarrassing to be staggering around the office
with that half-puckered look
on your face, and have to answer the inevitable questions. Soon enough, the answer is condensed to a "my-dick-is-asleep-yeah-yeah-I-know-fuck-you" in order to ward off the gales of laughter
In my case, at least, it turned out that when I got a new keyboard, I'd raised my chair seat a tad, and the lip of the chair was then high enough so that my feet couldn't sit flat. At that point, the lip of the chair was putting pressure on blood vessels on the underside of my thigh which apparently feed the crank some.
Ouch ouch ouch.