I was tested for polyneuropathy (PN) 8 years ago because I fell a lot that year. PN is associated with a few things, like the celiac I have, and MGUS. I subsequently tested positive for MGUS. I probably had MGUS for a few years, but launching myself down stairs had the positive (?) effect of detecting it. I had a BM biopsy this summer and now have 15% plasma cells and the 1q21 gain mutation, but no symptoms and doctors says still MGUS. MGUS has no known cause(s), just possible; it likely varies from one person to the next. https://www.melbournehaematology.com.au/fact-sheets/mgus-monoclonal-gammopathy-of-undetermined-significance.html https://www.cancer.gov/news-events/cancer-currents-blog/2019/mgus-multiple-myeloma-progression-risk

Doubt your diagnosis is from vaccine - MGUS tends to reduce immunological reaction- I had to get 3 vaccine for COVID to register any antibody response back in 2021 when they did such testing…