Hi - I'm A 54 Yr Old Female With A

Hi - I'm a 54 yr old female with a right bundle branch block. I've recently been waking up at night by the pounding of my heart. It's not all the time, seems to be intermittent - maybe running along the same week as my monthly menstrual cycle. I'm also about 9 months out from a bi-lateral mastectomy and latissimus dorsi reconstruction. I was diagnosed w/ DCIS and chose to have that surgery. About 4 months ago, my thyroid has also been diagnosed as having a left nodule + a little low T4 levels (.71 ng/dl) with hurthle cell changes. I go in for another check on that in Dec. My main concern is the heart thing because I've never experienced anything like it. I'm 5'5 - 129 weight -my normal blood pressure last month was 130/80 but is normally lower than that. The only drug I take is 20 mg Simvistatin. I know I have a lot going on but any ideas on why my heart would wake me up at night? Hormones??