Hello and thank you in advance for reading my inquiry and offering any advice regarding this issue. I have recently renovated a basement bathroom. This involved moving the bathtub drain about for feet from its preexisting location. The p trap still remains in its original location as it is the lowest point in the drain Line and is well below the concrete. The bath drain runs just slightly below the concrete floor then into the p trap. After installing the bathtub an the new toilet it was draining fine. A few days later after draining the bath bubbles aarose from the toilet and now it is barely flushing. I noticed when the toilet was off and the tub was draining the water level would rise in the toilet drain and eventually overflow. I also can run the hose into the toilet drain and it will drain freely. Recentlythere has been a lot of rain and the area in the yard with the septic tank is flooded and we can smell sewer gas outside when things are draining. I know ,there are a lot of factors here but any insight would be helpful. I really hope I don't have to break into the floor again.
Cheers, and thanks!
Adam

From your description, it appears that the system is "water logged" so there is nowhere for the water to go, so it acts like it would if the sewer were clogged, but in your case the only solution is to wait until the ground dries out.

quote: I noticed when the toilet was off and the tub was draining the water level would rise in the toilet drain and eventually overflow.


Sounds like you have a clog or as suggested a septic tank issue because leach lines are saturated with ground water.

Did you take any pics of your underfloor rough in plumbing? Did your system properly work before rains came?

check out this website: [septictankcare.net]

The easy answer is remove the cover of your septic system. If it is full to the top, your have drain field issue. If you can't see anything.... you have a restriction in the house drain line, that should be addressed.

Best Wishes