I Have Had Open Heart Surgery Also I Have A

I have had open heart surgery also I have a pace maker. The clinic I go to for my pace maker said I have Two wires one that go to the upper and lower chambers of my heart for a Time I was only using the upper wire now lately I'm using the lower one as well. I feel it when it kicks in every time . Why the change now to the lower and why would I feel it when the pacer works . It's uncomfortable to feel it.