What is happening to my calf muscle when I wake up to roll over?

Ill b sleeping and in the middle of the night I might wake up to roll over but then my entire calf muscle on one leg spazems and stiffens up and it kills like its getting stabbed and I can't move it and when it happend last night I felt my calf and it was as hard as rock. It goes away in about 15 seconds but its so painfull and my calf muscle would be sore for about 2-3 days after and it happens about once a month or I could even get it two nights in a row. I believew it my be a cramp but I have no idea because I don't exactly know what a cramp is meant to feel like. What is happening to me and why is it happening? What can I do to fix it? Is this normal and has it ever happend to you? please answer me.