Based on what you have described, IF the pain is constant and does not vary with exertion, I think is most likely from muscle and nerves and NOT from the heart.
As far as the murmur goes, I would make sure an echocardiogram was completed. Unless you have symptoms associated with exertion and or stress, such as pain, pressure, shortness of breath, nausea or eve just sweating, I would recommend a stress test.
stay on top of your Doc and let him know about your symptoms. If he don't listen find a new one. I have bent down many times and stood up with a chest pain. I'm sure its just a muscle since my 13 teen year old gets them too.