I believe I have an answer for the reaction to the vaccine. Back on May 10th, I caught the Covid B variant. It was my 4th case since April 2020. I’ve been warned not to get the vaccine because of a heart birth defect that could kill me from the reaction to it. Being the 4th case of it, I was going to take the chance and get the vaccine but after many hours of research, I learned the answer to your question. This information I found in May explained how in some people, when they get the first dose of vaccine, the symptoms were mild. Then when they got the booster shot, the symptoms were much worse. Scientist wondered why too and here’s the explanation and your answer. When you get the initial shot, the virus in not recognized by your immune system and begins to make antibodies. Then when you get the second shot, your antibodies do recognize it and go after it with a vengeance and the symptoms could be drastic. I became a long hauler after my 3rd case of Covid in March. I lost my taste and smell and feel exhausted all the time. I have a nonexistent immune system since birth but it must have made some antibodies because my 4th case was a Covid B variant and didn’t recognize it, so I got sick again May 10th and still feeling the effects. I hope this information helps to give you a reason why you feel like me without no vaccine shot.