Thank you for your query Water is important for every function in your body. Thirst and symptoms of dehydration like dry mouth, high colored urine, headaches indicate more loss than intake. Muscle soreness, on the other hand, occur due to lack of oxygen (increased demand during exercise), or Increased body temperature and can lead to dehydration in some cases (due to lactic acidosis) During vigorous exercise, or exercising after a break, the muscles tend to go back to its resting or basic state. When the demand on the muscles increase, it should, ideally, be matched by an increase in oxygen and blood supply. It also needs to have the required mass to carry out the tasks you set. If any of these is not met, it can lead to tired, sore and stiff muscles. The raised body temperature which may occur will settle in a couple of days. Stretching, hydration and light exercise till it goes away is advisable.. If you have done everything right and you have been doing the same routine for while now, you could be coming down with the flu. In that case, rest, hydration and proper nutrition should get you back on your feet. Remember that it is vital to keep your body hydrated to regulate it s functions. You should drink before you get thirsty. And sips throughout the day. Hope I ve helped. Please get back to us if you have further questions. Wish you good health