OK, first off, your blockage is somewhere below where the drain pipes for the toilet, sink, and tub combine. Have you snaked your drain yet?
For your septic system, what kind of septic system is it? Do you have a tank alarm, and is it flashing? How long has it been since you had your tank pumped? You should do this at least every three years - more if you have a big family that uses a lot of TP or if you use a lot of bleach in your wash.
Also, what exactly is coming out? is it white foamy stuff? If so, you're probably putting too many enzymes in there. My parents used to do that to our system when I was a little kid. Got a lot of white, foaming bubbles coming out of the tank. I used to love it 'cause the dog would go nuts barking at it
