In the last 2 months my vitamin d has lowered by 10 even though I've been taking a lot

[GTW]

Magnesium status is hard to measure but has a key critical and divalent effect on Vitamin D blood levels. It will lower excess/high levels.
The recommendation could be to supplement both D and Mg in moderate amounts and not psych yourself out if D is in the 35-40 range if no pathology.
>Magnesium supplementation increased the 25(OH)D3 concentration when baseline 25(OH)D concentrations were close to 30 ng/mL, but decreased it when baseline 25(OH)D was higher (from ∼30 to 50 ng/mL). Magnesium treatment significantly affected 24,25(OH)2D3 concentration when baseline 25(OH)D concentration was 50 ng/mL but not 30 ng/mL. On the other hand, magnesium treatment increased 25(OH)D2 as baseline 25(OH)D increased.Mg is essential in the metabolism of vitamin D, and taking large doses of vitamin D can induce severe depletion of Mg. Adequate magnesium supplementation should be considered as an important aspect of vitamin D therapy.Magnesium supplementation increased the 25(OH)D3 concentration when baseline 25(OH)D concentrations were close to 30 ng/mL, but decreased it when baseline 25(OH)D was higher (from ∼30 to 50 ng/mL). Magnesium treatment significantly affected 24,25(OH)2D3 concentration when baseline 25(OH)D concentration was 50 ng/mL but not 30 ng/mL. On the other hand, magnesium treatment increased 25(OH)D2 as baseline 25(OH)D increased.https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6693398/