Postpartum Depression after another baby?

I do not know why this happened but I didn't have it after my first child which was my daughter. It started after I had my third. So back then when just had two kids I loved my little boy I would play with him, kiss him, do everything even including my daughter I never left her out. But after my third baby was born I didn't notice till months later that I've been spacing my second out. Everything he does will irritate me like if he has a tantrum if he fights if he screams and it wasn't like that before what changed? My husband says he thinks I love the new baby more than him. But I think this is a real problem. Please no rude comments please I really need some advice.