Im 34 weeks pregnant and severly depressed, why?

i suffered from pnd after my son was born last january the doctor put me on fluoxitine and they did start to lift my moods now im at rock bottom again, i feel rotten, dont want to do anything, just feel like crying or sleeping all the time. i feel like i did when i had pnd, what has happened i was happy before now i dont feel like carrying on at all.