Breatfeeding is not contributing to your PPD or any underlying mental illness. What might be contributing is lack of sleep and stress. Some moms do get stressed out by breastfeeding, and depending on how your child nurses, it can cause more sleep deprivation (my son was a very extreme case, but he was an "inefficient" nurser--he'd nurse 90 minutes just for him to be able to sleep for 2 hours.... a part of it had to do w/ low milk supply due to serious blood loss from a c-section, but a lot of it was just him). However, I shudder to think what an utter basketcase I would've been had I not nursed--nursing releasing hormones that help the mom bond w/ baby and to feel better (seriously, this is scientifically proven, in multiple studies).
However, if you think that the sleep deprivation or the worries are complicating this situation, by all means explore not nursing. (My one cousin just worried, worried, worried that her baby was getting enough milk, and didn't believe it until she switched to formula and could see, quantifiably (sp) how much milk the baby was drinking.... it was making her crazy, not to know, even though the baby was growing fine).
I think, though, that you should be fine with continuing to nurse, especially if you'll be able to have the baby back home with you in a week. Just keep pumping, and nurse him every chance you get.
The first rule, though, is: Are you okay/happy. If you are, it doesn't matter whether you're nursing or giving formula... If you're happy, then the baby and rest of the family are better off.