From which vaccine? There are a number of vaccines, and it is expected for the body to "fall ill" a bit since the anti-bodies are being built. This is usually the case with all vaccines in general. I was an infact when I had received the various mandatory vaccines in India, but I recall taking the yellow fever vaccine when I was 32 and I had five days of fever

Similarly, I had a fever + 4 days of 20 hours of sleep after receiving the second Pfizer-Biontech dose.

Makes sense. The vaccine can cause those symptoms, but they could also be due to you catching COVID just before receiving the vaccine.

How do you know they’re from the vaccine?

He can't be sure of it, but it's the way it works, so its expected. Also, 2nd dose usually causes a stronger reaction from the body (you get the symptoms without the risks).

See https://xkcd.com/2425