It is backing up.
Do this firstStop using water. All of it. A system only keeps backing up while the plumbing is still being used, so every flush and every load of laundry adds to what is already sitting there with nowhere to go.
Then call us and tell us what you are seeing. Sometimes it is a pump. Sometimes it is a line, and a truck full of vacuum will not fix it. We will tell you which one it looks like before we roll.