I get PAC"s upon exertion in the Gym lifting weights, if I am not Breathing enough. If I relax a little breath deeply and walk comfortably they go away. Same thing if I am in high Anxiety, even at rest. If I relax and Breathe more, they subside. I have heard several others say similar things.
When going from rest to exertion, your Breathing might be inadequate, and your Oxygenation rate may be going down. If you have a Pulse Oxymeter you could test your Oxygenation rate when you exert yourself and get the PAC's. If you get really tired upon exertion, it could mean also your Heart is not pumping strong enough.