Past 4 mths have stopped exercising almost completely cos of lack of time. chest started to feel heavy recently. lack of exercise or sth more serious?

Exercise and Chest: With your age of 16, not exercising for 4 months should not lead to Chest Heaviness. I would look at what else may be going on . Is it worth with breathing, is it there all the time? You may want to seek medical attention if continues. AND..when feeling better. get back to exercise!

See your doc: is the chest pain always present? does physical activity or exertion trigger it? any family history of heart disease? what was your exercise routine and intensity 4-months ago? It is wise to discuss with your pediatrician and undergo a checkup to rule out anything more serious.