What Causes Flu Like Symptoms While Having Negative Results In Flu Test?

Just tested negative for flu, but have flu like symptoms. Vomiting, chills, fatigue. I have chronic cystitis and Dr. thinks that this may be the problem. Urine has been cloudy and foul smelling. Urine test came back 100% clean. Dr. said this could also be a flu false negative or another virus that isn t the flu. Prescribed 3 days of Cephelexin (?). Just looking for a 2nd opinion as to whether or not I need to be worried. I assume that if the bladder infection got into the kidney, there would be blood?? Do you take Paypal as a means of payment, I am always concerned about using my credit card via the internet.