I experienced this often when swimming in cold water during my younger days. I felt as if a bone is jutting out of my calf and it hurts like fuck. Have to hop for a while for it to go away.
The reason according to my swimming instructor/primary school teacher is the lack of sodium in my body. The cure was to drink salt water.
At 14 when my frens were teaching me swimming at Sg Poly Shenton Way then, I had to stopped over cramps at stomach, calves, thighs too. But not to that intense. Don't know why.