It’s happened to me a while back because of a likely electrolyte imbalance. I actually passed out in the hospital urgent care because I had gone in for feeling so shitty. Then they call an ambulance to take me from the urgent care doors to the ER doors across the parking lot. Of course the first thing they did before the ambulance even got there was put a saline IV in so by the time I got to the ER I felt perfectly fine and the tests didn’t find anything. Doc said I probably just had low sodium. That’s also where I learned for the first time that SSRIs sap sodium from your body which seems like something they should tell you when they put you on them.