I had something similar 30 years ago from a single tetanus shot. A baseball sized lump the doc drained and cultured. He said it was an aseptic abscess. We decided it was from the Thimerosol preservative. I knew I was sensitive to it because I had a reaction to contact lenses solution containing it. I’ve had flu shots for the last 20 years with no problem. I always check to make sure ther is no Thimerosal.
Interesting, thanks.