I have a question medical question and want to know if my doctor was right?

I was diagnosed with a slightly enlarged atrium, a mitral valve prolaps and atrial fibrillation which are under control by taking Cordarone. My problem is that when I pickup something or push something heavy, my heart start to beat rapidly as if my heart is trying to go back into a-fib. My doctor told me that a sudden increase in my blood pressure by standing up to quickly or by pushing or lifting can cause the blood to push up against the wall of the enlarged atrium causing my heart to beat rapidly and the medication (Cordarone) is keeping my a-fib under control by converting the beat back to normal. However, when I ask him to put it in writing, he refuses and will only write that I have a-fib. My question is, do you think he's full of crap? Could this really happen and how do I find supporting doentation to this effect? Thank you...