bipolar disorder depression

What causes breakthrough depression in bipolar disorder?
I’ve been on my present cocktail for more than 10 years now with few if any problems. Right now life is absolute hell–and believe me, I know clinical depression from the blahs–and I’m not sure why. I’ve got some outside stressors, but nothing unusual–so I’m wondering why now? What can I do about it anyway?
i found these sites………
http://books.google.com/books?hl=en&lr=&id=poZKLIB7vh4C&oi=fnd&pg=PP11&dq=breakthrough+depression+in+bipolar+disorder%3F&ots=K8iK7_yN4-&sig=z1bWIwx8Dy2QT_WK6NaE4B1SZ3s
http://linkinghub.elsevier.com/retrieve/pii/0006322395003797
http://cme.medscape.com/viewarticle/470126_12