And yes, in addition the very good advice above, if you start any new exercise it will use parts of your body you're not used to using. Obviously, boxing involves arm extension, which attaches to the chest, so you probably just feel sore there.
Hi there...when you have ruled out the physical then you have to work on the mental. It looks like your heart has been deemed fine so now you are left with the mental health anxiety. Has anyone suggested that you see a psychologist to learn cognitive behavioral therapy? Anybody that exercises is going to have an elevated heart rate. You can try wearing a heart rate monitor and try to stay within a normal heart rate for the type of exercise you are doing. When I do cardio, my heart feels like it is going to beat out of my chest but I don't have a heart attack and I'm 51. You are going to be fine but you do need to address the anxiety with a professional. Good luck.