Jennifer and I both have experienced this. Jennifer's experience was pretty bad. I noticed it only slightly myself, but it was there. We searched for over a year to find the answer. Jennifer subjected herself to a lot of medical tests and no one could explain it. At least they didn't find anything wrong
So we asked Mike Monroe, with push.tv, if he knew anything. He checked into it and said everything he found seemed to indicate insufficient carbs. It's important to drink more water and also eat a little more carbs with your meals, and even a carb such as an apple before a workout.
His explanation went into detail in an email he sent me and it made a lot of sense. I'll ask for permission to reprint it