There are hundreds of viruses that circulate that cause flu-like symptoms that aren’t the flu, that is, that aren’t caused by an influenza virus. (The flu shot targets influenza viruses.)
A lot of people who had flu-like symptoms did not have the flu:
About 4.1% of specimens tested at clinical laboratories were positive for flu. Since October 30, 2.7% of specimens have been positive for influenza this season.
When they did have the flu, it was influenza A (for this year anyway):
Nearly all viruses detected (97.7%) have been influenza A.
The flu shot was not very effective against influenza A:
“…these findings suggest there is low to no measurable benefit against influenza A.”
Here’s what Cochrane said about influenza-like illness (ILI):
Over 200 viruses cause ILI, which produces the same symptoms (fever, headache, aches, pains, cough, and runny nose) as influenza. Without laboratory tests, doctors cannot distinguish between ILI and influenza because both last for days and rarely cause serious illness or death.