You could be having a cardiac cause of chest pain
or gastritis/GERD or it could be due to anxiety
. One possibility is a cardiac condition as you have had a murmur in your childhood.But since your doctor has evaluated you and said 'you grew out of it' probably it can be ruled out. Another possibility is that since you knew you had a cardiac murmur in your childhood, you psychologically feel that you have a heart condition and probably that is why you feel that the pain is there.Yet another possibility is heartburn
due to acidity like GERD/gastritis.
There are many other causes of chest pain.Please visit a cardiologist for a thorough cardiac work up and if everything is normal , don't worry any more. Probably all it requires is some antacids/proton pump
inhibitors, if the problem is due to acidity. Or some counselling
if it is due to anxiety.
Hope you feel better soon :)