A few times in the last couple weeks, I have been woken up with pounding in chest and a spike in blood pressure (148/85).......I start trembling and get very cold. It goes away in about 15 minutes and I am able to fall back and stay asleep. I have been to the ER before because of my heart and I had chest xray, blood tests, and EKG done and all of them came back normal. What could be causing this??