Can postpartum depression happen this late?

Since babe has turned 12 months old, I've felt myself slipping.

At first I would only feel this way right before my period: The walls closing in on me, I resent my husband for going to work, I yell more often, I cry in stress.

After my period, I would be totally fine and back to normal.

But now it happens at least once a week. I don't know what's happening, and I feel lost.