I have the same thing, didn't know why though. I have had annual cardiac work ups over the past three years since I mentioned some mild chest pain to my doctor in 2005, he played it safe and sent me to a cardiologist. All my tests have always been normal and I asked her about these flutters when I take long slow breaths or even when I breath in quickly like when you have the sniffles and she said it was most likely due to a compression of the compartment your heart is in. Since my tests were normal she was not concerned.
I don't let it bother me anymore, but if this is something you have not discussed with your doctor, you probably should.
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