Amlodipine, a calcium channel blocker may cause hyponatremia due to its natriuretic properties.  Amlodipine-induced hyponatremia is quite rare.  A case reported earlier was also taking Amiodarone in addition to Amlodipine.  Amiodarone can cause hyponatremia due to the SIADH mechanism.

Another possibility is that reduction in the entry of calcium ions into the smooth muscle cytoplasm will result in reduced contraction (i.e., relaxation) of the muscle and thus overall a vasodilation.