Magnesium blood level was 2.5. All other electrolytes normal & normal kidney functioning. Not on any supplements/meds containing mag. Is this bad?

Serum useless. A serum magnesium level is not very helpful. The body keeps serum magnesium under very tight control at the expense of tissue/bone levels. Check an rbc magnesium level instead to get a better picture of tissue levels. Ideal levels in the upper half of reference range. Most people are low.