Why do I get sudden and unpredictable leg cramps?

This morning when I woke up, almost as soon as I sat up my left calf cramped up and it was extremely painful. Even now, the hypertension is still lingering, making it difficult for me to walk on that leg without shooting pain. This has only happened once before, and it was back in December. Does anyone know why and how this happens and if there is a way to prevent it? Much appreciated.