The fainting is most likely caused by a drop in your blood pressure. It can be brought on by exertion/heavy work. Since your resting BP is already low, and then you don't breathe how you should be...Its a nasty combination.
The blood in your body isn't sending enough to your brain. Perhaps if you are in a 2-point for 20 minutes, the blood is not flowing how it should be due to muscles flexed. Then when you sit back down and release your legs (the blood swooshes down into your legs and causes a lightheaded feeling)
Now, this ^^^ HAS happened to me before working out. Sitting is a chair position on a wall for 5 Minutes. When you finally release your legs, I can feel the blood returning to my legs and will have a "fuzzy" brain moment for a few minutes until my head is clear. Never to the point of fainting thought.
When you start to feel dizzy,you need to elevate your feet. (Impossible to do on a horse I know...)
If you are otherwise healthy...There may not be much you can do for it. Do you ever have problems outside of horseback riding? Like dizziness getting out of bed?
Did you bring this up in your physical? Understanding Fainting -- the Basics Posted via Mobile Device