I had PND too though it took some time to realise/admit it, DS was over a year at the time.
We had a rough start from labour to leaving hospital (no excuse I know) much as I loved him, there were times when I felt very resentful, (sounds awful) mainly when it came to feeding him and the times when I was feeling crap I tried to do my best before leaving him in the cot and I'd hibernate in the study and surf the Net for God knows how long .
I remember once in his pushchair, when he was over a year, shaking the chair with him in it, I then caught myself and my feelings and walked away.. I felt the worst mummy ever and it makes me cry to think of it now.
I would tell DH that I was awful and what made it worse was that I used to work with children and NEVER treated them like I had my OWN child I convinced myself he would be an insecure child etc and felt incredibly guilty... then I started to listen to DH and other people, even strangers and noticed they were complimenting ME on what a happy little well adjusted boy I had .
I then in time became more kinder to myself with my thoughts about myself, and now DS has started school and is VERY happy, social, kind little lad.. am pregnant with number2 and am praying this time around I wont have any repeats.. hopefully things will be better regarding circumstances, e.g a better start to the arrival, and will have DH around, as last time he was having to go away a lot.. it made me really respect my own mum and lone parents who did the upbringing of children all by themselves, because I felt so pathetic.
You sound a great Mum, and you are definitely not alone!