He hasnt regressed so much as he's beginning to use his actual strength again. The poison hasnt taken full effect in the least but he realizes what it will eventually do and it's caused him some anger. Being someone who's rarely pushed towards distress by physical circumstance, this is something he wont take lightly. He eminates such an aura, that even when he's faintly disturbed, all who are unprepared will be overcome with an exagerrated and powerful form of whatever emotion he projects. Typically he's far more in control than in this specific circumstance, but it's been nearly one billion years since last he's utilized more than ten percent of his strength. Right now hes riding on about 15-20%, but thats still a hell of a lot when he's consciously whithholding much of it's residual effects even now. Also, the grey man threatened to steal something very precious to Pahn, and with the poison being something that could make such a threat possible in the future, Pahn's mind is racing. 

I'll send you plot details in the PM's, sorry I never discussed my plans with you on that one! I definitly should have bee thinking to tell you all about it xD