Postpartum depression

Can postpartum depression happen 6 months pp? Lately I’ve been so moody, down, and mean to my boyfriend. I’m with him the most so he sees it and I just feel like I’m angry with life. He is the kindest and helps out so much but I just nitpick at everything.. we fight constantly because of it. I’m not really sure what to do and we have talked about it and I tell him I’m so sorry and I’m having a hard time being “happy”