It can be really anxiety or some minor side effect of your immune system doing the work. Also we should note that note everything happening after vaccines is because of the vaccines , specially when it is way after.
I have the same after my first shot of Moderna 6 days ago. I have random on and off pains on my upper back and that made me very anxious that it could be something with my lungs but no, it's not even when I breathe and it is very random and mild.
My first 2 days after the shot were a complete panic, I didn't sleep waiting for symtoms.
Because of my severe anxiety, random pains are expected anyway but then you don't know what is what and your anxiety goes crazy.
Well, first of all, relax. As a person suffering with severe anxiety I can tell you , I have muscles twitches , chest pains and what not . And I always had, it was not the vaccine.
If you have Instagram try to follow real doctors and scientists that explain everything about the vaccines , including that the mRNA and spike protein in them don't last in your body more than days or weeks, and this is exactly why people are going for second and even third shots. It just doesn't last in the body forever. The effects that can happen are: 1. Straight away after the vaccines (first 15-30min) and mostly alergic reactions or 2. In the first few days because the immune system is doing the job and you will probably feel something. Some people have strong immune responses and can have weird effects but again, it is most likely to happen in the first few days and it's not forever.
Nothing more can really happen and there's plenty of healthy happy people after the vaccines.
I really understand your fears because as I told you I have severe anxiety and I'm even re-thinking the 2nd shot due to this only, it is my issue.