It's a hard thing to explain. Even to explain to your self is virtually impossible.
It starts just below your ribcage, and expands to fill your chest. The feeling is how I'd imagine swallowing a ballon and it slowly inflating. In optimism, I'd like to say that its a light feeling. However, a darker, pessimistic trail of thought suggests hollow, numb, nothingness. Unfortunetly, my pessimistic voice overpowers all else at the moment.