Hey Dale,
When you take a full breath in and fill your lungs, there is slightly less space for the heart. So, with filled to the brim lungs you can expect heart to be a little snug in there and that might be what you are feeling. That or your lungs push it a bit when they are filled. I think this is normal. Given that our pericardiums are left open post OHS, I would expect the heart to be a tiny bit louder. At this moment your heartbeat might still be a little strong, just like mine was, and it might feel a little weird. As the heart remodels it will feel better and be calmer.
My basic understanding is that heart is a muscle, and by putting it through various ranges we rehab it to work better with new parts. Heck it will rehab/remodel on it's own over time, but in my head if I give it a bit of a challenge, even if a small one, daily, it should remodel sooner.
How is everything else going? Sleeping well? Eating well? Walking daily?