Post Partum Depression

I already have 3 kids and I never experienced ppd for my 1st 2 kids. I think i'm having one now, my mom thinks the same. I don't know, not really sure. I do get tired and stressed out easily, I get emotional and cry easily (which is unusual for me). These are some of the s/s of ppd. Books could not say how long an individual stays in this condition. I'm just being positive that I wouldn't be depressed for a long time. It's not in my personality to be sad like this. I'm a happy person and I like to stay that way.