I had a bout with passing out about 6 years back. I passed out about 3 times in a year and half for no reason. The last time was in the bathroom and I hit my forehead on the towel cabinet. Blood all over, cut me right between the eyes. My wife made me go to the ER. Same treatment as you got, heart monitor and the whole works. No results. They came to the conclusion that it was a combination of dehydration, sleep deprivation, and stress. All caused by caffeine overdose. I guess 2 pots of coffee, 3 to 4 Pepsis, and 18 hours a day at the gun shop were just too much for the blood sugar to keep its natural balance. I took their advise, cut back to 3 to 4 cups of coffee, 1 Pepsi, and lots of water. Started getting more sleep and working less hours. I haven't had another spell since. I have since sold the gun shop and started working a normal work week for another company. It's been 5 to 6 years and I haven't had a single fainting episode. Sounds similar to your experience except I have never been on any meds. I am also 13 years younger.
May God be with you, and get well soon.