I know I was a mess after my third child, most of it was due to lack of sleep. I literally thought I was losing my mind. Going back to work can also be very stressful. Try, if possible, to find some time each day for yourself, even if it's 15 min in the bathtub instead of watching t.v., or being on here. My daughter is 4 now , and I"m prego with my fourth, and from time to time I end up in tears for God only knows what reason. Hormones can play a major part , also. I hear it takes a good year to get back to 'normal' after having a baby, so your body could still be adjusting.
A few things I have tried, other than trying my hardest to get more than 4 hours of sleep, lol, is taking good vitamins daily, and trying to eat better, as in, fresh fruits,(apples) and veggies, and avoiding as much white flour and sugar as possible. (I am a sweet freak, so I know how hard this can be.) A good 10-30 min walk in fresh air also does wonders for the mood.
I hope any of this helps. Hang in there, you'll make it thru this. God Bless and take care.
L.