Since this is coming from all your fixtures, sounds like you either have a plug in your mainline to the septic tank, or the septic tank isfull and/or you have a septic drainage problem.
If you have not had your septic system pumped ina long time, this may be the best place to start.
Next bet would be to have a plumber snake out themain line.
Have you noticed any septic bubbling up in youryard?
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