Pain under my left breast exactly where my heart is, pain when laying down on left side, does not go away, i have asthma, makes my breathing very hard?

Not sure: There could be many things that cause your symptoms. The good news is that a heart attack in a 21 yr old woman would be highly unlikely. Usually is a musculoskeletal problem. However, i recommend you see a doctor and get it checked out. The only way to say for sure is with a detailed history and physical exam and maybe a few specific tests. Get it checked out.