Some people legitimately have a bad immune reaction to vaccines, for a wide variety of reasons. It could be a specific allergy to some component of the shot, or a number of other factors. This isn’t unique to the COVID shot. It’s relatively rare, but everyone who is unlucky to be one of that minority goes online to talk about it, so there this inflated sense of it happening that far outstrips the real world occurrence of it. I’m sorry that you have had bad reactions in the past. That sucks. Thankfully that’s not the experience of most people.