I've been getting sharp sudden pains in my sternum area and also just to the left of my sternum. Serious?

Over the past few days, i've experienced sudden, sharp onsets of pain on my sternum and also just to the left side of it. I also am experiencing a slight amount of tightness and discomfort along with the pain. I am only 20 years old and I don't smoke, and like to think of myself as not the most unhealthy person out there. I've been experiencing pain in my left under arm and down my left arm too, but usually only after waking up in the morning. I think the arm pain is caused by me sleeping on my side, and i'm not sure whether it is related. I'm very worried and I'm incredibly anxious since this has started happening. What could it be?