<s>[INST] Every now and then I will feel my chest tighten and my heart starts pounding extremely hard. I can't breath and I begin feeling very lightheaded. It usually only lasts a few seconds and does not happen too often. Any ideas as to what could be causing this? [/INST] Hello and thanks for writing.I can understand your concern and would try to help you in the best possible way. The symptoms you have described are some what red flags and need a thorough workup to get to a cause. A few conditions that might cause them include cardiac arrhythmia, angina, anxiety or Blood Pressure issues. You need to be evaluated with a complete medical history and physical examination. You may also need  few tests like BP measurement, CBC, KFT, ECG, Holter study and others to rule out cardiac ailments.I suggest you consult a physician and get yourself worked up for the problem. </s>