Huge hugs LN! It's normal to feel like this when you are sleep deprived and hormone are all over the place! I've had PND with some of my babies and not had it with others, and have felt like this in the early days of all.

You and your DS have already through a lot! I shall tell you what many people have told me (especially when DS3 was in the special care nursery)
Be easy on yourself, you are physically, mentally, and emotionally exhausted!

It's quite common for babies to have day and night mixed up, in fact DS3 did until he was about 3 weeks. I spend most of his first 2 weeks walking around in a daze and crying at nothing!
Truthfully sleep when he does, if he sleeps for 5 hrs you sleep then too.