Experiencing sharp burning pain lower pec sternum area during cough. No pain resting, breathing. Recent bout with flu. Muscle/tissue inflamed?

Get chest X-ray . There many things that can cause these types if symptoms. There is a chance you could have a pneumonia or it may come from your chest wall. See your doctor for a physical exam. She or he may order a chest x-ray for you.
May be. It is likely that, but because of your age it would be advisable to have it checked out.