Halfway through my first AT session, while holding a pose half-sitting I nearly fainted, had the sensation of blacking out preceded by a long moment of nausea. We took a rest, I got flushed and began a cold sweat. Ok, I was freaked. My teacher was surprised and said it was odd for a young person in good physical shape and health to experience this but said not to worry, that it sometimes happens.
This is very common. There's no one explanation for why it occurs, and it doesn't seem to matter how old or young a student is, how healthy or 'balanced' they are - it simply happens.
I imagine it has to do with the inner ear and some form of recalibration of what constitutes 'normal' use. This could happen with even the slightest move away from a person's habitual state, and is most likely to occur when a teacher is guiding a student. I would only start worrying if it happened repeatedly.
Having said that, "Holding a pose half sitting" does sounds as if it could conceivably have affected blood supply, generally.
These are only my views. I'm afraid I have no authoritative answer, although I'm sure you have little cause for concern.