It could be the acid reflux or an esophagus problem.
I wish I had an answer for you.
Three months ago, while super-relaxed, sitting my couch, I suddenly felt as though I was losing consciousness, my heart started pounding (but not racing), my vision went blurry, I felt like I wasn't getting enough oxygen (but not shortness of breath), muscle trembling, whole body shivering, nausea, and diarrhea.
It lasted just a few minutes.
I went to a walk-in clinic right away, but my vitals all tested normal and, by then, my symptoms were greatly lessened (coming in decreasing waves, the rest of the day). I was told it was too vague to diagnose with a physical exam.
The 3rd episode (and worst to date, at that point) landed me in the ER. It took 2 more ER visits before a doctor actually considered a possible cause, rather than just dismissing me. That doctor thought "heart".
Alas, all my tests, includng heart monitors and echo. have come back normal. So, now the theory-du-jour is that I may be experiencing severe hormonal imbalance reated to perimenopause. I am almost 50. I have no idea if that's the cause, partly because my periods are as regular as clockwork, and I have never had a hot flash or night sweats.
For now, I'm in wait'n'see mode.
Best of luck with your diagnosis.
Same to you. So sorry that's happening, but at least its not your heart. :)