Don't worry about the cold making you shiver. It won't trigger your atrial fibrillation.
Stress, high blood pressure, alcohol, arterial disease and caffeine are your worst enemies and the occurrence is slightly higher in those with a heart valve defect (like me).
Most arrhythmia is benign, if disconcerting, and if your Doctor has confirmed it is harmless, fish oil/oily fish, Magnesium and Garlic are the best things to emphasise, as they keep the electrical impulses more constant and keep the blood vessels nice and clear.
I hope this helps, it certainly did me if that reassures you. Feel free to ask any other questions.