There is no evidence that taxotere or carbaplatin have any adverse effects on the heart. Herceptin is known to rarely cause some heart changes, but not specifically atrial fibrillation.
Atrial fibrillation is actually very common with age. Most people get it for reasons related to high blood pressure, valve disease, coronary artery disease, lung disease, alcohol use or thyroid problems. If one of those were to blame, you and your doctor would most likely already know that you had the risk factor. But up to 1-3 out of every 10 people with atrial fibrillation had no risk factor for Afib.
Be optimistic! There are lots of options for atrial fibrillation treatment and in many patients it can be fixed.