What can be causing horrible chest pains?

My grandmother started having horrible chest pains that radiated from the ears to the throat and down to her chest. She took Nitroglycerin at home and it relieved the pain for about 15 minutes and then came back . Eventually she went to the hospital and they did an angiogram on her which showed that all her vessels are clear and open. She felt better and was released from the hospital and then the pain came back again. She took Nitroglycerin again and drank Maalox hoping maybe that it is gas. She had a heart attack 6 years ago and has 6 stents put in which are all clear of anything. What can be the cause of this horrible pain?