I remember this on time I got a huge cramp in both of my legs after a nap. I woke up and I flexed my legs to get the blood flowing better. The next thing I know, however, was excruciating pain in both of my legs. It felt as if all of the muscles in my each of my legs bunched up into a ball the size a pin and everywhere there wasn't muscle, pain replaced it. It hurt so much I fell off the couch I was previously sleeping on. The pain lasted for a good two weeks after it, no matter how much I stretched.
The experience left me paranoid of another one for a year or so.