I've had heartburn all day. Now I have a sharp, sudden pain in my left arm under my shoulder. Am I having a heart attack?

Unlikely. Heart attack is unlikely for you as you are a young female unless you would have other conditions such as severe dm, rheumatologic autoimmune diseases or preexisting childhood heart disease. You do have to see your doctor as this constant heartburn, and this sharp pain which seems more like a nerve problem now has to be explained and diagnosed. Good luck.