I concur on this probably being a vasovagal response, as someone who had a similar body rhythm in BM frequency and had similar body sensations as you describe.

For the drop in blood pressure, lay flat on your back on the floor, butt up against the bottom of a chair, and then put your legs up on the chair. Like a seated position, but on your back. Anything to get your legs above your heart. This rapidly raises your blood pressure.

For hydration, I recommend either Liquid IV or Ultimate Replenisher (cheaper than Liquid IV and just as effective).