What Causes Breakthrough Bleeding After Taking The Pills For A Week?

Hi, I've been on the pill for over a year now. I've never had breakthrough bleeding before but this month, a week and a half in to a new package of pills, i have been experiencing some bleeding. Does this mean that I am unprotected if I have sex right now? The doctor said i should wait it out until my next package and if it continues she will put me on a stronger pill.