My plumbing started working in reverse with the outflow backing up into my washer. (Like all normal Rhode Islanders I have the washer in the basement.) Thank the Goddess for good plumbers.
Now everything is flowing in the right direction and we will not, after all, become a methane resource.
Kmareka.com 
As my father used to say, “It’s always somethin’!” Glad you have a good plumber. If she can change shower heads, please give me her number as I need some help with that.