Fair enough! My original comment was more brief than precise...
If it helps the original commenter, and anyone else reading who's suffering from similar issues:
The umbrella term for syncope and fatigue when sitting up or standing is orthostatic intolerance (OI), and it's commonly seen in Long Covid and other post-viral illnesses. Both POTS and orthostatic hypotension can cause OI.
To the original commenter, there's a good chance you've already been tested for irregularities on a CBC and CMP, but a thorough doctor would generally also order tests for your thyroid, testosterone, iron, B12, and Vitamin D levels. You should probably also get screened for diabetes, autoimmune, and adrenal issues. These labs could help identify simpler, treatable causes of your OI and other issues.
But if all that is normal, testing for orthostatic issues like POTS and orthostatic hypotension would be extremely warranted for you as well. And it really can be as simple as an at-home test! A lot of doctors will only diagnose POTS or orthostatic hypotension with an official tilt-table test, but with the rise of Long Covid many specialists are willing to diagnose and treat patients based on simpler and cheaper at-home testing. And the initial treatments are often as simple as drinking additional fluids and electrolytes and wearing compression garments.
It took me several debilitating years and at least a dozen doctors to get diagnosed with POTS and orthostatic hypotension, and if any of them had suggested I buy a BP cuff and spend 30 minutes doing a lean test it would have saved me an awful lot of trouble...