I'm not a doctor but I'll tell you what the doc finds when I have a serious cold or a bout of the flu. I get dehydrated. My kidneys complain about it so my back hurts where my kidneys are. Clear liquids like water or ginger ale are good. A bottle of Gatorade is good couple of times a day to keep your electrolytes up.
On the other hand. Laying around can be troublesome on your back. Get up and move around from time to time to keep your muscles active. Even though you feel like you've shot at and missed, s**t at and hit, get off your ass.
Go see a doctor. Coming on line to ask is asking for trouble.