Hi!
I really doubt that you are pregnant, given that you are using a regular method as well as Plan B. It is very common for Plan B to cause irregular bleeding, so I'll bet that is all there is to it. I ALWAYS recommend taking a pregnancy test though, so don't skip that!
If you are consistent with your Yaz, you really shouldn't need the back up. However, if you just want that extra bit of security, remember that Plan B is a hormone, and it will very likely cause side effects such as breast tenderness and spotting.
Good luck!
Dr B