Why do I get sharp pain in my left arm & rarely my right, then my heart feels cramped. I'm 21 & my doctor checks my blood pressure regularly its fine?

More detail? This is unlikely to be due to anything important at this age. It sounds musculoskeletal (?Sport related) or stress related. Nonetheless if it persists further history and tests are needed.
Benign chest pain. Brief episodes of chest pain occuring at rest or without exertion is common in young people and is usually not related to the heart. If there is no connection with activity and no breathing problems it is probably benign. An ekg and possibly an exercise stress test should be done though.

Related Questions

Male 36 years old with previous heart attacks and stent procedure who has home blood pressure reading of 157/109/132 on left arm sitting. 160/119/128?

High blood pressure. I recommend that you see your internist to re-evaluate your blood pressure control. Your numbers are not that good and you risk further compromise to your health if you don't. Read more...