My boyfriend and I have sex with me standing/leaning against a countertop, sort of like doggy style if I was vertical from the waist-down and horizontal from the waist-up. (Imagine a person at a not-so-strict 90 degree angle) If we only do this for a few seconds, I'm fine. If we do this for a few minutes and he finishes inside of me, I move to the shower to clean up, and I begin blacking out after about 30 seconds or so of standing fully upright. We've never done this for more than a few seconds where he does not finish inside of me.
My vision goes black in a sparkly way, and I immediately sit down in the shower to avoid falling over. Then, I have to drink some water and stay in the fetal position for a while before finally gaining the ability to move to the bed. Any idea what's going on here? Does this happen to any of you? I've seen a cardiologist for heart issues before, but they've never found anything wrong.