Hi Jeffy14, there is another explanation aside from OI.
OI is orthostatic intolerance, and most of us have that. POTS is Postural Orthostatic Tachycardia Syndrome, and many of us have it. Both involve loss of blood pressure to the brain, and pooling of blood in the legs. I have OI but not POTS.
My problem with shaving is concentration. You have to stand and concentrate and coordinate your actions for some minutes. If prolonged, this can trigger post exertional malaise, especially if you shower before shaving.
Showering, particularly hot showers, will also worsen OI. You might try having an only warm bath instead if this is a problem for you.
Bye
Alex