i think it can take up to 18m for PND to be diagnosed, im reading a fab book called depression after childbirth and it focuses on the hormonal aspect of pnd rather than the emotional. it suggests that during pg your progesterone levels are so high that when you give birth the drop is too much for your brain to understand/cope with, women who bf are at a greater risk of developing PND later on because while you are bf-ing your hormone levels are stablised and havent experienced that dramatic drop, when you stop however the same thing occurs.
you should see your gp, its more common than people realise, most women experience some level of pnd at some stage and the relief once youve told someone is huge. HTH