Breakthrough Bleeding?

I've been on the pill for over a year, though I've changed pills twice... this is the second time, unfortunately I missed a pill and followed the directions such that I took two pills the next morning when I realized that I missed a pill. Now a weekish later I'm having "breakthrough" bleeding, which I've never had before. Is it normal to experience mild cramps with this? This is also the second day... when is it likely to stop or should I see a doctor? Any information would be great.