With me, the relationship between flutters and coffee is complex. Caffeine alone doesn't seem to do much. It has to be coffee – but if I have too much coffee, then have another or some other caffeine, I get flutters. The effect also lasts many days, rather than the few hours caffeine stays in your body.
I think it's something else in coffee (it's full of complex chemicals, theobromine for one) which causes problems in conjunction with caffeine.
Just try staying off the coffee, tea, chocolate or coke for say three or four days. I find that if I overdo coffee one day, even one cup then causes me trouble. I have to 'reset' myself with a few days off coffee then I can drink plenty again. I say plenty, I'm tlaking two cups max.