Your drainage is stopped up and needs to be cleaned out by a plumber or if you know someone with a "rotor router" or a 'snake" you can do it yourself... get the clog out and the sewage should be able to drain fast enough without backing up into your tub
Sounds like the toilet and the tub share a common drain and there is a blockage below where the two meet. There is a possibility that you have an access below the bathroom where you can send up a plumbers snake and open the drain. If not, you can try blocking the drain and overflow in the tub and plunging the toilet. As a last resort or maybe just to save a lot of time, call a plumber or Roto-Rooter.
About 15 years ago something similar to this this happened to my in-laws.
They had no idea they had a plumbing problem, and by the time they did know it, it was too late! They came home from dinner one night to find that the darned toilet had "blown out" all over the bathroom! YUCK!!
They called a plumber as soon as they saw the mess, and he told them, after checking it out thoroughly, that the root system for the willow tree in their neighbor's front yard had made it's way into the water source.
The in-laws got to spend 4 or 5 days at a hotel for that one, but what a mess they had to clean up!
Chin up and good luck! Let us know how you make out!!
