I have experienced these symptoms as well. No cause was found other than some muscles being extremely tight and inflamed. The inflammation can spread into your cartilage around your rib cage and cause heart-attack like pains.
At your age, after been checked by your GP, a serious heart problem is unlikely. Your experience of increasing discomfort could very well be explained from your increasing stress level. Try to stay relaxed and wait what your cardiologist has to say, seems the best option to me.