septic odor

My half bath just underwent a remodel, which meant replacing toilet and sink. Now, on certain days only, I get a strong septic/sewer smell. (I have septic.) It comes and goes. I don't know if I have a septic problem or a plumbing/install problem. How can I figure this out so I know whether to...

I am getting a mild septic odor in my downstairs bathroom. It is only a half bathroom, has a toilet and a sink. There is no other bathrooms or plumbing areas that have the same odor. I have a newer home, built in 2008 that is on a septic system. A little about the system. 1000 gallon spetic...