It would have been better if you had mentioned your age and given any history of the past illness. Anyways, I will try to help you. From your symptoms it is clear that you are suffering from general weakness. You need a " Master healthcheck up". You are suffering from hypotension
, you also have electrolyte imbalance
( potassium, sodium, mineral depletion). This is causing you pain on your back and thighs. You are dehydrated and should take re-hydrating fluids, soups and juice. I suggest you to visit a doctor in emergency room.
You need to be examined clinically, investigations and blood tests need to be done to rule out anaemia and diseases like viral infection, influenza
It will be good if you get admitted in a hospital and get treated through IV lines ( fluids and antibiotics ) to get relief in a shorter time. Have a nice day, take care.