It sounds like it could be a kidney stone. Do you feel the need to pee often, even if you just went? does the pain get less (even just a little) when you go to the bathroom? If it is a kidney stone, Flexerol will not help, but something with codine should, but not always. Try Advil or something that is anti-inflammatory. Drink plenty of water. Incase it could be a pulled muscle, I would take it really easy this weekend, lay around and dont do anything stressful or too active, see if that helps. Also alternate ice and moist heat, see which helps you the most (personally I respond better to ice). If you still hurt when Monday gets here, go to your doctor. Kidney stones can be common with CFers because of dehydration and high protein needs (a urologist told me this). I hope you get feeling better.