It might be completely normal, but I think it will get better with a lot of support from your midwife. Are you still under midwife care?
I know my lowest point was at 2 and a half weeks, so days 3 and 4 are not always the worst. Maybe for you, today will be the lowest point and it will all improve from now 
Please talk to your midwife . Whether it is the start of PND or not, you need more support than you're getting, and your midwife should be arranging it for you.
I really hope you feel more positive soon. It is a huge adjustment to make when you become a mother, and I think we expect to take it in our stride (afterall, we've had nine months to get used to the idea, right?). I think it is perfectly normal to feel shocked, and to take time to adjust.
With regards to coping when your husband goes back to work, you will. My husband filled the fridge with yoghurt, milk, cheese, ham etc and the cupboard with chocolate, biscuits, crisps, heinz 'taste of home' pots (wonderful - please get some! You microwave them and have a complete stew meal in 2 mins!). You can then concentrate on yourself and the baby, with easy food on hand when you can manage it.
Also, pop over to the Breast and Bottle Feeding section for advice on bf pain, and/or ring one of the helplines (details will be in the feeding section). They're normally really helpful.
Well done for making through this first week - please ask for support with the next few [smeile]