I have been quite lucky when it comes to bcp. I am currently on my 4th different kind of pill in a little over a year. I went from necon 35 to alesse to ortho-novum 777 to ortho-tricyclen from july 2002 until now.
I have never experienced breakthrough bleeding with the pill. The switches I made were consectutive, with never missing a month in that timeframe.
That being said, it is more than normal to experience the breakthrough bleeding.
The only thing that has me wondering is you only switched from a brand to a generic drug--essentially, you are taking the same thing. The breakthrough bleeding shouldn't have occured by you taking generic over brand.