postpartum depression?

I gave birth about 6 months ago to my beautiful son. He is my whole world, and I would honestly do anything for him. But... I'm not sure if I'm suffering from postpartum depression... during the first 6 weeks after giving birth I constantly thought about hurting myself or even killing myself..  but after about 3 months that went away.. and now every time my son cries a lot and won't be quiet.. he's fed.. he's changed.. and I have no idea why he's crying I get soooo mad at him.. now I'm going to get a lot of people judging me that's why this is anonymous.. I need advice not your opinions.. now postpartum depression runs in the family.. my mom admitted to me when my brother was younger she got so mad at him she shook him really hard (didn't kill him though) so when I get mad I feel like hurting my son.. I just want him sto STOP crying.. but I think of how bad my mom feels about what she did to my brother.. I put my son down and walk away from him... I'm sooooo worried about hurting him and doing what my mom did to my brother..  there has been one or two occasions where I've patted him on his butt harder then I wanted to... I want to talk to someone about it but I'm afraid they will take away my son... I love my son more then anything in this world.. hurting him is the LAST thing I want to do to someone who looks at me with trust and love in his eyes... and I know many of you want to probably report me to child services so I'm remaining anonymous... before you start becoming harsh please remember I have feelings to and the mean things you say won't make me feel any better...