There is evidence that MGUS is a risk factor for both bacterial and viral infections Monoclonal gammopathy of undetermined significance and risk of infections: a population-based study. This may have been exacerbated by your Rituximab treatment.

This is just a comment (I am not intending to second-guess your health care team), but I am surprised that they recommended Rituximab treatment.

Usually MGUS is just monitored with no treatment. It can progress to multiple myeloma or a similar lymphoproliferative disorder but this is rare (about 1% per year) - regular monitoring should pick this up.