I Am Being Woken Up Several Times Each Night With

I am being woken up several times each night with heart palpitations. Also, my chest and back have a burning sensation. I have not had much sleep the past few nights and I feel horrible. I have a demanding job and I need to be mentally sharp. I was on medication for several years for generalized anxiety disorder, but I stopped taking it in the spring. I have been to see the cardiologist who performed several tests and I wore a halter monitor for a month, but he can't find anything wrong. I am 53 years old and I don't drink or smoke. I am very frightened.