Hiya, i get this all the time it mostly happens when i am in bed and move my leg suddennly, i am in such agony i have to walk up and down the bedroom sometimes for 15 mins or so then get back in to bed very slowly in case it tightens up again, very annoying at 3 or 4 in the morning!!
My doctor told me it was a lack of salt in the diet, but i dont use salt never have so looks like i am stuck with it, i would say it happens roughly once a week. Oh and if its any consolation i DO cry sometimes! ; )