It is common for most vaccines, that a person will become ill for a short time after receiving that vaccination.
But when a person receives a flu shot, we know they become sick with a mild case of the flu. At least that is how I understand this.
In the case of the Mrna shots, you are not being injected with a milder or dead version of the Covid virus.
So in this case, what exactly is it that people become sick with for a few days after receiving the "jab"?
Symptoms will dictate the likely etiology. Some of the immediate reactions are likely down to severe allergic response to some ingredient in the shot, like generalized inflammation and even anaphylactic shock. Very rapid drop in blood pressure, fainting often fatal without immediate intervention.
After Spike production autoimmune responses seem to be common. Immune system starts producing (or may already have if previously infected) anti-Spike antibodies. Those antibodies will attack the Spike and the cells that are sprouting them. I like to think this as the "fuck you and the horse you rode in on" response. Autoimmune hard to diagnose and treat.
Biggest problem is we don't really know what is in these injections and the FDA wants to hide information for 55 years or more. Clearly not all of the lots contain the same stuff. If you let them shoot you up with this shit, you are in unchartered territory, never before encountered in the course of human history.