Postpartum depression

Can someone please explain to me what this is? people keep telling me that I have it but everything I’m reading says things about having thoughts of harming a child or not feeling connected with them and in my case that’s not it, I love my son so much.. theres not one thought of harm. But I am so disconnected with myself I’m always so down and sad feeling alone and far from happy. I can’t seem to find myself my son is going on 4 months and I have been feeling like this since he was born, I just don’t feel like myself anymore I find myself crying all the time I just wanna be happy with me again but I can’t seem to find me