Questions about cramping

In 2 weeks of regular swimming (2-3x weekly), I've gotten myself to over a half mile already. My speed has increased noticeably. From a tip I already picked up out here, my freestyle form is much more rythmic, and my *** stroke has is starting to take on the early signs of a butterfly. But I've been having some really nasty cramping issues in my calves. I mean horrific pain and knotting. What's a good way to combat that? Is it the water temperture that causes it? (the pool I swim in will never be mistaken for a jacuzzi) Any tips or advice will be greatly appreciated.