Salt
Could it be not enough fluids? I know when I'm getting dehydrated, I always take a pinch of salt - it helps you retain fluids. Our bodies sometimes crave things that it needs without us even knowing it.
I remember when I was pregnant with my son, the doc asked me all sorts of questions about whether I was craving/eating paint chips, paste, etc. I laughed, but he said it was a common thing to have a natural craving for that kind of stuff if your body lacks the nutrients and minerals it needs....