It could be a heart problem. The breathing thing makes it sound like it could also be respiratory issue, or muscle cramps
due to being out of shape. It could be a type of asthma, I used to get mild asthma
attacks only after I exercised. It felt like a very tight throat, difficulty breathing
, and alot of coughing and weird chest sensations.
But... since this is really bothering you, you should see a doctor! It's definitely possible for young people to have heart problems, as apparently you already know. And with anything this uncomfortable and worrying, it's worth it to get an expert opinion and see if there's anything that can help you.