Woteap I found my maternity leave very difficult. I went back to work 3 days a week when my DS was 8 months old. I was lucky that I could afford to gradually settle him into nursery over the previous 1 month.
I used that time as some time for me to get my head together.
Can you afford any childcare? If you are planning to use a nursery then you could start off with a 1/2 day per week maybe?
Don't worry about labels of "PND". You are recognising that you are not quite happy with how you are at the moment.
I think it is so normal and Mumsnet is definitely the place for you to be to find that you are not alone and lots of other people have struggled with similar thoughts.
Some suggestions for you would be:
Listen to this.
Have a read of this.
Look into Baby Led Weaning - this book.
Have a think about some activities you could do with your DS.
Do you know about treasure baskets? Or see this book.
I also like all the ideas from here.
Do you have a sling or a hipseat? I used to like taking my DS to the shops on the hipseat and it was a lovely way of feeling more connected with him.
I also second the idea of osteopathy. Even if just to get to talk to someone who is interested in your DS and his development. GPs just don't have the time to talk about general development and temperament etc.
The osteopathy can be very good for feeding problems. (I believe it helped my DS with reflux problems).
BTW - well done on ebf for 4 months. There is a whole world of "feeding guilt" out there on the www if you look for it!
Try out some of the suggestions on this thread and take on board what people are saying about it being so much easier when they get to even about 9 months old.