My first thought is dehydration. As Jey100 said, it could also be hyponatremia (low sodium). You could try adding a gram or two of salt (or my preference Morton's Lite Salt, which is half sodium chloride, half potassium chloride) to a liter of water with just a few drops of lemon or lime juice.

Another alternative popular with cyclists to prevent dehydration, muscle cramps and hyponatremia is pickle juice, just make sure it doesn't have added sugar. Substituting a tablespoon of apple cider vinegar for the citrus mentioned above would give you about the same thing.

What you don't want to do, if there is a possibility of hyponatremia, is to drink large amounts of plain water, which further dilutes the remaining sodium.

There are also commercially available, calorie-free, electrolyte drinks or powders.

If sipping on a liter of one of the mixes listed doesn't relieve the muscle pain or makes it worse, you should see your physician for some tests to determine the cause.

Of course, don't use electrolyte drinks if you are on sodium restriction or have a medical condition where it might be contraindicated without checking with your doctor.