I don't have any practical poly experience to contribute, but I have two children and I do know that newborns and toddlers have a huge impact on relationships and inner life -- in fact, both times, I didn't really get my normal/sane/sexy/balanced self back until baby was about three. Mileage will vary on the timeframe, I expect, but I know that in the year(s) after each baby's birth I wasn't even coping very well with a standard mono relationship despite the fact that RugbyMan is my best friend and a good guy and hot and all that. It just isn't easy for everyone to bounce right back after pregnancy and birth and having an infant/toddler in the house.
I guess what I'm saying is don't judge yourself for how you're feeling right now, Lace. Personally I don't think I'd recommend experimenting with poly and coping with a baby at the same time, but given that it's already happening, just remember that you might not feel as strong and together and loving right now as you once did or as you will when your baby is older. There are a lot of hormones and stuff to deal with, not to mention lack of sleep -- it will pass. I'm not saying you'll feel differently about monogamy/polyamory but you will feel able to process the issues more confidently and cope better with taking steps toward the future you want to have.
And if you think you might be dealing with some PPD, even minor "baby blues", please talk to your doctor/midwife!