English is not my first language so I can't visualize you lifting a manifold, I'm not even sure what that is in this context, but... how did you lift it? When I lift something from the floor to a level above my head, as I'm rising, my vision turns black and everything starts spinning for a moment... they told me it was because of low blood pressure... but I don't know, it doesn't happen every time, and it isn't causing me much problems.
Perhaps someone with a better medical background can give you a better answer... I just thought these 2 cases might be related.