Oh Nickel, no wonder you are upset, it's a very stressful situation you poor thing.
I am pregnant with DC3, and look back 8 years ago to after DC1, and can so easily see I was suffering with PND. I think - at the time - I figured depression was lying in bed in a darkened room. And I did get out and about to baby groups, and got dressed, and DC1 was fine, so I must be fine.
But to feel this stressed is not right, and it's not your fault: you have a lot on your plate, your hormones are probably still all over the place and sleep deprivation does not help. A lot of us on MN have been there, and that's what the support out there is for: make an appointment with the GP, and as another poster says, write it all down to prevent you minimising it when you get there.
And have you thought about contacting Homestart? It's a brilliant organisation that is set up for mums feeling like you are right now (and me 8 years ago) - someone could come, sit with you and entertain the baby for a couple hours so you can have a bath/ read a book/ sort out your printer - whatever would make you feel better. If you search for them, you should find one in your local area.
You poor thing, it is rubbish, and it can be hard to feel like you can talk about it (at least I found it hard at the time), so well done you for coming onto Mumsnet, and the next step is getting an appt with the GP. Let us all know how you get on. xx