PLUMBING QUESTION? Why is water leaking into my house from floor when sinks and tub drain?

Ok, so when i allow water to drain from either my kitchen sink, bathroom sink or bath tub, stinky water leaks from floorboard area between my kitchen and bathroom. Two days ago i noticed water leaking from underneath my dishwasher. Also noticed some moisture and mildew under my kitchen sink (right next to dishwasher) so today i pulled the dishwasher out figuring something was loose. Well there is no visible leak between the dishwasher connections or underneath the kitchen sink. So i cleaned up the stinky disgusting water that had puddled and peered back behind my kitchen cabinets and it's just aweful!!!! It stinks and i can see build up and mildew and possibly some mold. anyways, theres about a foot of space between the back of my kitchen cabinets and my outside wall and i noticed that when i run water from either of my sinks or my bathtub, water leaks in from what looks like the wall!!! i hope its not a sewage backup!! could it be something loose in the plumbing located outside underneath my siding??? Any suggestions as to what would be causing this would be greatly appreciated!!