I strongly recommend you get your nutritional status checked. Low B12 can really, really contribute to depression and many of your symptoms. Many of us (1 out of 3) do not high enough levels of the enzymes necessary to properly metabolize B12 in the gut, so even if it's in your prenatal vitamins, you may not be absorbing - this applies to other B vitamins with B6 being really, really important for a sense of wellbeing.
Also, you don't mention whether you breastfed, but often that can minimize the impact of PPD since your hormones return to normal in a much, much slower manner. It is some of the dramatic hormonal changes that can also contribute to PPD.
And, finally, please have them check your thyroid and not just TSH. Have then check Free T3 and Free T4 (FREE is very important as that's what's bioavailable). Also, have them check for thyroid antibodies. It is very, very common for folks that are borderline hypothyroid to become more hypo during and post childbirth. If you have autoimmune thyroid disease (which is what most low thyroid people have), then your thyroid levels can vary dramatically, thus this is something that you should check now and post childbirth and really should be the first thing any doctor tests for before they ever Rx anti-depressants and again, they need to measure much more than TSH, which is actually a pituitary and NOT a thyroid hormone.
Personally, I don't think PPD is necessary inevitable for anyone and I think your regular OB is very wise. Also, if none of these doctors are testing your thyroid or nutritional status, you might want to find a really good PCP that knows about these things. Most OBs know virtually nothing as they just don't have time to study this subject.