False Start repeated

Got off to a good start last week.
On Monday I did a Cooper test and achieved 2.64km, my best yet.
Then Tuesday morning I did 10 minutes on the stairs and in the evening a very good 20km run with Fatih.
By Wednesday morning, however, I was feeling worn out.  Wednesday evening I only managed 5km. I have rested since then.
This bug I have that is sapping my energy is proving difficult to shift.
I feel better today, so we go again.