There is no one here that can tell you because we are not doctors and nor should we suggest you may have a heart issue. That is for a doctor to diagnose and only for a doctor to diagnose. If the pain is diminishing, I can tell you it sounds like you might’ve pulled a muscle during one of your walks or your bra isn’t fitted properly or you have some kind of indigestion. That’s causing pressure in your left breast area. However, if I was you, I would want a doctor to make sure I am not having a silent heart attack and have some discussion going forward about these kinds of pains so you can sleep well at night. And as well, if it’s not serious, but if it is a concern that a doctor can proactively take care of it before it gets to a serious heart attack that might even cause death when it had the opportunity to be avoided! Good luck and please see a doctor.