The adjuvant used to increase the immune system response is a big culprit in this. Each vaccine has a different adjuvant, a few years ago they used aluminum based ones. Mercury may be another adjuvant, I am not sure. Your body is supposed to mount a sufficient immune response, so that can be considered normal (soreness, heat at the site). That being said, I never take the flu shot. I take Zinc every other nite and a multi with copper every day.
All but one flu vaccine on the market does contain mercury. In addition dog vaccines still contain it, though if you insist you can get a mercury-free Rabies vaccine for your dogs, but you have to insist that is what the vet gives you. Don’t forget these vaccines are grown on things like eggs and that is why they ask people getting flu vaccine if they are allergic to eggs... the polio vaccine was contaminated with a Simian virus back in the 50’s/60’s and cells from that virus is appearing in cancer cells these days.