Exercising does increase your heart rate and breathing, etc. which can be similar to a panic attack; could easily be confused for the feelings one has then. Could just be that you're worrying about something else you don't want to face so are projecting it onto your health. Your health is okay so takes the psychological brunt of the projection. I would work on it "literally" (you're afraid of being afraid -- future fear which you can't do anything about either way so are "stuck") and just rebut it with logic.

I would see if you couldn't read about "health" subjects rather than the opposite. The Centers for Disease Control is very good for accurate information on how to stay healthy.

http://www.cdc.gov/HealthyLiving/

Do you have a therapist at all, someone to talk to about your fears? I would try to think back and see, from just before this happened, what happened to scare you and talk it over with a trained therapist.