Hi brief answer muscle spasm Detailed answer thanks for trusting us as your medical expert since your all reports and ECG came normal there could be two probabilities and both are extreme one could be a local muscle spasm or costochondritis which can be relieved after symptomatic treatment other could be any cardiac cause for that you have to look for a cardiologist for ECHO and TMT depending upon your detailed history and after physical examination. I tried to keep ot very simple hope this will help you Regards