It can be a lack of salt, lack of calcium, or even magnesium (I think). Drinking lots of water can help too. I've had a couple of cramps, especially when I'm stretching in bed. I have to remember not to stretch my legs in a certain way or I can feel a cramp coming on.

Are you still taking multi-vitamins? I think someone suggested once before eating a lot of bananas? Can't remember tho.

Hope you only get the occasional cramp and it's not too painful