What is the physiology of it? For the past 4 hours I've been convinced I was having a heart attack (although I am 21 and my heart is healthy). I actually felt chest pain and shortness of breath. I've gone to the ER before and been told my heart is fine. Yet I actually felt pain.

So, how is that possible?

[CANDC]

Here are some articles that may be of interest:
Psych Central - Search results for Chest pains caused by anxiety

[CosmicRose]

Because when the adrenaline is pumping, your heart is going really fast, there's also a degree of muscle contraction in your chest, hyperventilation and shortness of breath makes it worse, an EMT once told me its the over abundance of oxygen molecules in your blood stream (something like that, I'm sure it has a name but it's escaping me right now) I think its co2 or something. Anyway, if you're breathing heavy and having shortness of breath, there's either a lack of or too much of that in your bloodstream which can cause anything from muscle contractions to a host of other physical symptoms like slurred speech, passing out, etc.
People often report feeling like they're about to die, feeling like they're having a heart attack or even a stroke when this happens, I know I did.
It mostly has to do with your circulatory system. Any strenuous activity on the heart can make it hurt. This is also tied to the flight or fight response, the body either fights or prepares to die (not really but it feels like it).