Hamstring pain

While swimming, I was on the recieving end of some very sharp hamstring cramps. It froze up and wouldn't let go for quite a while, and swimming afterwards was a little difficult/painful. Why would this happen while swimming(I barely kick, my kick is just a bare minimum to keep my legs up) and what should I do to get it to loosen up(30 minutes later, its still very tight...)
I've stretched as much as I can, and spend some hot tub time.