For the past week, my chest has been hurting consistently. there's numbness in my left arm, sharp pains, a burning feeling, and stiffness. any idea?

Possible disc injury: Being 21 years old makes it unlikely that this pain is cardiac in nature. More likely you have a disc injury in your neck which is causing this issue. A c6-7 disc injury frequently causes chest (pectoralis) pain which can radiate into the tricep and arm. It can also cause numbness/tingling down the arm to the index and middle finger. I recommend you see a spine specialist.