What could be causing extremely rapid heartbeat and dizziness when waking up?

I have just started (past few months) experiencing almost daily episodes of waking up in the middle of the night to extreme dizziness (when still lying down) to the point where I feel I am about to p out. I can feel my heart beating so fast that it's almost vibrating and I can feel it skip and will lose my breath. I also get very nauseous and clammy when this happens and sometimes I have numbness or sharp pains in my hands and feet. The few times I have tried to get out of bed, I ended up on the floor not being able to move for several minutes until my heart rate slowed down some and I wasn't as dizzy. I went to the emergency room for this recently and my heart rate was at 177 when I came in. After giving me some meds to bring it down, they couldn't come up with an explanation. I have a hyperactive thyroid but it's under control with medicine. I also got a holter monitor test which showed tachycardia but my doctor doesn't seem to think anything else needs to be done. The only way I can stop these episodes is by taking Ativan. The next day I am still dizzy and nauseous. I feel Ativan is just a temporary fix and something serious is going on that is being ignored. I am in my 30's and normal weight so this heart stuff is really scaring me. My Dr. is reluctant to refer me to a cardiologist for some reason. Does anyone have suggestions on tests I should insist on having or know what this could be? I am not sure why it's happening so much when I wake up in the middle of the night. Thanks