Hello,
I understand your frustration with doctors. I have had anxiety for many years now and my sis is going on 11years of having it.
Even though you may not feel like you are having anxiety 24/7 the symptoms of anxiety can come out of no where for no reason even when you think your fine. My sis has had the fluttering and had ever test possible and they all came back normal. After many years of the fluttering she still says she has no idea when it will flare up. Sometimes it will bother her a month or 2 and then go away, then it comes back a couple months later.
If the test came back norm and the other doctor thought you were fine, you probably are, but Go check with one more doc and don't tell him/her about the other doc visits and see what they say. If everything comes in okay then I would say it is anxiety.
Anxiety is mean and creeps up and does scary things to your body and makes you feel like the world is done for, but it always turns out okay.
best wishes!