I don't think you should worry unless it gets very heavy and last for quite a few days.
You are right, the medications you are/were taking will not effect your pills or your cycle. However, stress can. Even if you are on birth control. So, I would guess that is your problem.
Break through bleeding is very common (but inconvenient). I, personally, have experienced this a few times in the past. I could have break through bleeding just a severe as my normal 'cycle' during the sugar pills. My doctor told me that unless it is very heavy, or happens on consecutive months, not to worry. However, if you are uncomfortable with this, please call your doctor. Either your doctor or nurse will be able to tell you if you should be concerned.