My heart doesn't like being pregnant either and I'm reasonably fit. However my resting heartrate is always around 100, no matter how fit I am. It is too high and it worries me what it's going to be like later in life, but like you I've had all the tests and nothing comes back abnormal.

However in my last preg I was diagnosed with hypothyroidism and one of the symptoms was an elevated heartrate. Maybe you could ask about that?