There is certainly a difference between 'just' a flu and something worse (sinus infection, and so on).
The body notices when there is something more to it than just a flu.
That's the point were you get yourself a nice appointment at your 'Hausarzt' (family doctor, sounds wrong tbh) and let yourself check thoroughly. If somethings wrong, you'll get your medicine. If not, well just get yourself some rest. Our social systems and working rights give you the freedom to be ill for a long time and not needing to worry about losing your job.
Maybe in the US you are so used to getting your painkillers and hardcore medicine for a simple flu (, and so on) that you cannot withstand some pain.
Most dentists don't prescribe you painkillers or even antibiotics after a wisdom tooth removal, even if it was a operation and not just pulling it out. Yes it hurts as hell, but just stay at home for 3-5 days, get some rest and live happily ever after.
Working from 7:30am to 5pm. Cannot understand the German work ethic and the often seen German workaholics.
You work to live, you do not live to work!