On a more serious note:

• Flu gets worse in the afternoon, PEM usually better
• Flu & friends usually cause some symptoms PEM does not – like soare throat, running nose, coughing, fever, while typical PEM-symptoms might be less prominent.
• For me, it just feels different. I’m just fighting my first severe flu-like infection since CFS started, and it feels completely different even when the words to describe it are almost the same.

ymmv.

Gute und rasche Besserung für alle!

Valid medical test for this problem