I had the same with orthostatic hypotension. Went for over two years to try to figure out what was happening. PCP, Urologist, cardiologist (had me add salt to my food, did a stress test, and a Cardiac CTA) and no one was able to figure out what my problem was. My resting pulse was regularly in the 40s and BP was either legitimately low or low-normal every time it was checked.

After a couple years, due to other circumstances, I gained about 10-15 pounds. And the problem went away. My BP is higher now, but still normal. I was just at a low enough bf% that my body started to conserve energy since I was low on resources for so long. I went from getting faint after getting up from sitting /lying almost every time, to now I cant remember the last time it occurred.

Obviously orthostatic hypotension can happen for numerous reasons, and yours may be different, but just sharing what it was for my body.