W.L.
I don't know how much post pardom you are ( you said you are nursing) Have you started a normal cycle yet? Have you had regular periods? It could be your body trying to regulate it's self and adjust to being a mom and trying to adjust to the meds as well. There are lower doses of things they can give you for depression. I was on them at a few points in my life so I would perhaps talk to your doctor and see if you could possibly change. I don't think you need to check your thyroid or anything that serious just yet. Just try making a few adjustments and talk to your doc.
Best Wishes,
W.