My adult (39) son is, among other things, a sought-after basso singer. Full voice, 3 ledger line Ab. When in HS, he noticed that low Russian choral basses would close their eyes on the lowest note, and reopen them just in time for the cut off. Why? - he wondered. When he grew those notes, he found that it made his skull vibrate - and affected his vision, making him nauseous. Betcha you have something similar.