what kind of magnesium are you taking? anything with -ate on the end is more readily absorbed by the body. i take magnesium citrate and usually take 650 mg at night.
the multivitamin is of little use -- they mostly have too much of what you don't need and not enough of what you do. you might want to consider using a potassium/magnesium combo though and other than loose bowels unpleasant side effects from magnesium supps are relatively rare.
a weird feeling where you describe may be more related to digestion...
As I ate the oysters with their strong taste of the sea and their faint metallic taste that the cold white wine washed away, leaving only the sea taste and the succulent texture, and as I drank their cold liquid from each shell and washed it down with the crisp taste of the wine, I lost the empty feeling and began to be happy and to make plans.
– Ernest Hemingway