I have big problems in my old house. Started with kitchen sink not draining, so I plunged. It finally went down and ended up in my bathtub. now the toliet is either overflowing or yuck comes up in the bathtub. I have a septic tank. I tried using a utility pump and drained alot of water out of it. Still having the problem. House is over fifty years old. Pipes are cast iron. When I use my sink or toliet it rains water from a trap in the basement. HELP.
I would start by hiring the local septic tank company to pump the tank(s), and explain the situation to them before they start pumping, so that they can be attentive to any backflow from the field, which would indicate a problem there. In the mean time, I would also check the vent stack(s) to make sure that it is not/they are not obstucted by a bird nest or other debris. Again, these are starting points, and if they did not solve the problem, I would begin locating cleanouts, and snaking the system within the house to the tank(s).