Sudden onset of body aches, fatigue and chills. on day 3 of feeling incredibly weak and run-down. complete loss of appetite. could this be flu?

Yes: Sounds like flu especially if there is fever. One should see their dr if they are not feeling stronger within a few days after the onset of such an illness since other possibilities do exist.

Flu?: Yes, it may very well be the flu. Early diagnosis permits early treatment with a product like tamiflu (oseltamivir). Also, early detection may allow for treatment to reduce the severity and communicability of the virus.