Why is this happening to me calfs?

Alright. This has happened a couple times before. I've had cramps in my calfs and they really hurt, and when I rub them, they go away, but they keep coming back. It happens last night in the middle of the night and then when I woke up for the day, and then all day, it feels like I've worked out for like 4 hours. It hurts that bad when I walk. Anyone know what it is? Or what's wrong?