sound like the drain is clogged you could call a plumber or you can rent a snake and do it yourself if you think your able. there is a washout near the septic tank,that has a threaded cover,just remove the cover and run the snake int the pipe, if you get a sudden flow of $#%@ water through the pipe, the drain should be clear. also you mentioned a septic tank, if it has a sewage pump, and the pump goes bad, the sewage in the tank will build up, and will eventually back up into the house. if it does have a pump, there is a high water alarm,did it go off?the alarm is a grey box that will have a red light on it, is it lit? it also has an audible alarm.