I get this weird mild pain in my left arm that comes and goes, is that a sign of a heart attack ?

Highly unlikely: If you are a 21 year old generally healthy male it is unlikely that left arm pain is an "anginal equivalent" I.e. Due to inadequate blood supply to heart muscle.. Things that would make it more likely to be cardiac in nature would be if it is mostly brought on by physical activity. Even if your arm pain is exertional it is unlikely to be cardiac in nature unless u have a strong family history