Are you increasing your water intake when you do? That sounds like dehydration. Creatine takes a large amount of water to appropriately process, and during the loading phase, your body is pushing substantially more water into your muscles. Anecdotally, if I don't drink something like an extra half gallon of water a day while loading at 15g/day, I show symptoms of dehydration. And I'm already drinking somewhere between a half gallon to a gallon a day.