When I run or walk fast and for long time, I have a bit chest pain but not always. Is it associated with Propranolol 20 I take everyday? Is it MVP?

Chest pain & MVP. Are you on the Propranolol because of episodes of irregular heart beat related to your MVP? Folks with MVP can experience some chest discomfort with exercise. Make sure you discuss with your heart doctor. Could consider stress test to see if chest pain with exercise might be related other problem. Good luck. .
Probably not. In all likelihood your chest pain is musculoskeletal in nature. MVP usually causes non-exertional chest pain. Propranolol is unlikely to be source of your discomfort. MVP in my experience is overdiagnosed. I would suggest that you engage in exercises that strengthen your core. By doing so you will likely ease the discomfort in your chest region. Best wishes.
Chest pain. At 25, I would not be that concerned. However, if you do have palpitations of feelings that you might "pass out" when you are active I would be more aggressive at working you up. I do not know why you are on the Propanolol, but it is a therapy for MVP. Have you been worked up for MVP, Echocardiogram?,