Hello, and welcome to Healthcare Magic!
You are describing palpitations (being aware of the feeling of the heart beating, or beating rapidly). Because you are young and otherwise healthy, it is unlikely that this problem is being caused by a serious heart condition. But it still warrants a visit to your
primary care doctor to be examined. It is very likely that
smoking is causing this problem, or that it could be related to periods of
anxiety. In any case, even if the smoking is not the cause, you should stop smoking now before it begins to have negative effect on your heart. Also, if you do any illegal drugs such as cocaine, crack,
methamphetamine, or other "uppers" they could be causing this symptom.
Keep exercising, stop smoking, and go see a doctor who can examine you for signs of heart problems.
I hope I have provided you with some useful information, and wish you the best of health.
Sincerely,
Dr. Kinney