Post natal depression support?

I just dont know what to do?? I have a 7 year old, 26 month old and 12 month old and I feel like I'm losing my mind. I'm pretty sure I suffered PND with my first but was in a bad relationship and thought it was just the stress from that but now I know differently. I had no trouble with my second child as I have a wonderfull hubby to be and my ...little girl was very easy baby. even after having my 3rd I have been fine up until recently. For the last 3ish weeks I feel terrible, 2 minutes of my 2 littlest ones crying sends me nutty, I start crying and scream at them to "shut the hell up", I actually have to walk out of the room because I'm terrified. I would never harm my children but honestly feel like driving (by myself not with the kidlets) into a bloody tree just so I never have to litsten to it again. Getting a doctor in my town is impossible, I cant even call my health nurse as I have no home phone or credit on my mobile, hubby is away for work (uncontactable). I dont know what to do??????????? please help me :-(