Why is it that any little activity I do causes chest pain?

[jeremy]

Backstory...I played soccer for 16 years. I went into the marines (made it all the way through boot camp and then had a stress fracture which gave me a medical discharge). I was always in great shape for soccer and was pretty active in sports. About 2 years after bootcamp I played in a basketball game with the company I worked for and collapsed on the floor and was taken to the ER...where they said I had a mild heart attack. I went to my doctor a few weeks later and he said he thinks it's probably exercise induced asthma. Now I have never had problems up until that point. I was very out of shape when I tried to play in the game. I'm 26 now and want to get back into running shape but I went outside to run a mile and only ran about 1/2 mile before it was too much pain to bear and I walked the rest of the way. Why am I having this problem? I desperately want to get back into running shape so I can run with my friend (who runs 5 miles at a time sometimes).