I only take magnesium with a Calcium/D combo. (that would be 999 mg cal/334 mg mag X 2 daily. 300mg doesn't seem a high dosage....
Calcium will also ease muscle spasming...
I hugely ditto you get your vitamin D checked.
Have you had your sodium and potassium checked as well? what about B vitamins?
I also agree to get in to see a rheumie.
q