Can Excedrin lower blood pressure? My BP is usually high. So today I've taken Excedrin for a migraine and my BP is way lower. Relation?

Yes. Pain and nxiety raise BP and increases the heart rate. They are tressors produing a catecholamine response. Feeling relief may produce the opposite effect and that could lower bp.
Blood pressure. Very likely the pain releif, there is nothing in excedrin that would lower bp.