The idea of vaccines is to a trigger an immune response. A mild fever for a day or too after the shot is normal and not particularly dangerous.
Exactly, and the more foreign the antigens, the more the immune system will respond in most people.
For example, what is thought to have killed so many people in the Spanish flu of 1918 was that it was so novel, the resulting cytokine storm had people drowning in their own lung secretions.
This is a well-known innate immune response, 'secretion' or 'fluror', is that the immune system tries to 'wall off' the infected area via secretion, which is why infected or injured areas are often swollen.
If you encounter a seriously foreign antigen in your lungs, well........
If it wasn't for ITU beds now, we would probably still have a lot more deaths of this nature.
Anyway, if you feel really crap from the tiny amount of antigen that is given to you in the flu vaccine, it could mean that the strong immune response (as it is the immune system response that generally makes you feel really rubbish, not the virus) indicates that the virus was very foreign to you and you are lucky to encounter a tame version rather than the full fledged beastie.
Tell that to all the people who say that 'I felt really bad after the jab', that they are probably likely candidates for death in the coming global pandemic if they can't even handle the micro litres of flu epitopes. Of course, unless they are the lucky ones to go down first and get the ITU beds.
Anecdotally, I used to feel really sick when I first started getting flu jabs, super sore arm as well. Now that I have had so many flu jabs, I have to push on the injection site to even notice that I have even had a jab.