Wow....if I was normal and shook my head for half that time, it wouldn't be good
. What was the point of that exercise for an actor? To play the part of a dizzy person
You can find lots of real people to do that on this board (lol) without practicing. I definitely wouldn't try that again.
Subs on this board may have an explanation for this and may reply, but my thought is shaking your head rapidly as you stated I would think would be like an injury like wiplash. It could have done something perhaps to the fluid in your ear or who knows but just maybe knock crystals in your ear loose (BPPV).
It is good your showing improvement. Was it your instructor who told you it would go away or a doctor? If it continues and you didn't see a doctor, I would go see one.
Hope you recover quickly.