It sounds like you may be experiencing angina (chest pain) due to cardiac ischemia (lack of blood flow to the heart.) Chest pain is caused by a lack of blood to the heart thus causing a lack of oxygen to the heart. This causes pain. If the lack of blood flow and oxygen remain too long, cardiac cells will die.....called a myocardial infarction (heart attack.)
Several things can cause lack of blood flow to the heart such as plaque, blood clots, atherosclerosis (narrowing or hardening of the arteries,) etc...An angiogram can detect narrowing of vessels or presence of plaque or atherosclerosis however, the blood vessels are able to dilate and constrict by themselves. If the blood vessels have an episode of constriction, you may experience chest pain due to the decreased blood flow to your heart. An angio may not detect cardiac ischemia if it is due to episodes of constricting blood vessels because the vessels may not be constricting at the time of the angio. Make sense? I hope I am explaining this okay to you.
Nitroglycerin causes the arteries to relax and dilate (widen) which causes an increase of blood and oxygen to the heart. The fact that the nitro helped your chest pain is significant evidence that you were experiencing a true cardiac event (angina.) Angina can lead to a heart attack if the heart goes without blood flow for too long and cells die. Cardiac enzyme blood tests can detect if there is damage to the heart from angina. Depending on how high those enzymes are determines if there was an infarction (cell death / heart attack.)
A stress test would be a good test for you to have. Your chest pain can be controlled with medication and you should be monitored by a physician. In the meantime, continue to go to the ER whenever you have chest pain so you can get appropriate treatment (nitrates, oxygen, etc...)