You got it. Orthostatic hypotension secondary to dehydration, overheated room, overtight shirt, etc.
The "flex your knees" comment is right on. It's common knowledge that if you stand up straight (knees locked) for too long a period, you are vulnerable to fainting. Something about the blood being partially cut off by the straight-leg posture.
Didn't see Kimmit's episode, so I don't know if this was related to posture.