From your information it sounds like you've been checked by a cardiologist, an EP that specializes in heart rhythms. That's about as good as you can get. If your doctor is confident your heart is fine, that you're just having the occasional little electrical blip, the trick is to convince yourself that he's right.
My heart does PAC's and PVC's every day, average 1-5 a minute. When my heart decided to pick up the pace to 15-25 a minute, I wasn't happy and called my cardio. They checked my heart, said it was PAC's and PVC's and said I would be fine. After 5 months, I was fine and my heart settled down. But like you, when there was a sudden change, I wanted to know why. With ectopics there is often no reason why they act up. Irritating little monsters. LOL
Just remember the more stressed and fatigued you get from worrying about them, the more likely they'll stick around in greater numbers. Stay rested, stay busy and let's hope the medication change gives you some relief.
Thank you for your question. I defer you to the heart rhythm forum that has participating members that can identify with your condition and provide support and advice. Take care,
http://www.medhelp.org/forums/Heart-Rhythm/show/92
Ken