I Have Rapid Heart Beat Fairly Often It's Cause Is

I have rapid heart beat fairly often it's cause is really unknown as it can wake me from a sound sleep. I have had two heart surgeries to replace my aortic valve. I take Lipitor and wondered I this was part of the cause of the rapid heart beat. I have also recently lost 20 lbs so weight should not be an issue. Thank you for any advise you can give me.