What Causes Throbbing Pain In Lower Back Post A Bout Of Flu?

Hello, a week ago I started having a fever near 103, sore throat and cough, nausea, and a loss of appetite. Since then, my temperature has dropped down to around 99.5-100.5 and my sore throat has for the most part gone away (not the cough, though). I went to my local Doctor and tested negative for Strep, and the Flu, but she said she still believed it to be the flu. Yesterday I started to feel a throbbing pain that I believe are in my kidneys (or lower back) and the pain was consistent regardless of how I was sitting or laying. My Mom thinks it is because I have been bed ridden with the flu the past week, but I just wanted to be sure it isn t anything more. Also I have been drinking a lot of fluids! A couple side notes are that I am 19 years old, and at the start of the school year I was hospitalized with Rhabdomyolysis with my Ck level peaking a little below 70,000. Thank you for taking the time to read this! -Chase Robinson