Hi COgirl! I've been getting the same thing except I feel like I traded my "sadness" for rage, frustration, & anger. I have been trying to deal with it by actually letting myself get mad, really letting myself feel it. I think a lot of my problem was that I'd keep my anger in, or at least at bay, for whatever reason (be it guilt, feeling like I should control myself, if you're annoyed just get over it, etc.). I found that by letting myself really non-judgmentally feel the rage was no different than having a good cry. But, I promise myself to feel it, then let it go completely. No hanging on to it for the rest of the day, week, or month. That was just making me a ticking time bomb.
Also, postpartum depression can kick in any time within the first year after giving birth, your hormones are definitely still up and down right now, especially if you are breastfeeding. Good luck!