Urination
Peeing a lot makes sense on a liquid diet... liquid in, liquid out but pain could be so many things... like throwing darts giving you an answer to that one.
My guess is it's just your body in ketosis (basically a metabolic shift from using energy from carbs to fats, ketones expelled in urine and bad breath is sometimes an issue) liquid diets like Optifast basically kick you into that quickly...
but I'd ask a Dr. about any pain to be sure.
HTH,
~Michelle "Shelly"