About 10 years ago, my igm was very, very low. hasn't been tested since. can it have come up or will it always be low?

How low was low?: Selective IgM deficiency is a very rare condition, and the mere fact that you are fine means that you have an intact immune system despite the possibility of low IgM. That was a single reading, could have been affected by a disease you had then or a medication you were on, readings differ with age, many normal people have 2SD readings less than normal range.If you're otherwise healthy don't bother