Fevers, chills, muscle pain etc can be strong immune reaction.

Migraine and muscle pain can be same or reacting to preservatives etc in the vax or other stuff. Slightly abnormal.

The weird stuff is women noting it messing with them hormonally - period changes, breast pain, moods etc. Significant hormonal issues are not part of a typical vax.

Very very rarely vaxes cause nerve damage or othere severe injuries - probably won't see people reporting that on reddit because they will be in the hospital.