Is it normal to have borderline low immunoglobulin m (igm) levels? Do they fluctuate that much? ...Or nothing to worry about? Doc wasn't worried.

Don't worry. It may fluctuate but i won't be concerned with an isolated low igm, let alone a borderline low one. The immunoglobulin levels , like any other products produced by the body, do fluctuate a little with time but usually within a narrow range in a healthy person. Some levels may elevate in the presence of infections or allergen exposure.