Sewer Vent Problem?

The problem I'm having is when I flush my upstairs toilet the down stairs toilet bubbles up. There is just air coming up no sewage. I recently replaced the upstairs toilet and this started about two weeks later. The septic system seems to be working fine and was pumped this summer after it backed up do to an unusual amount of rain that flooded the entire area. I also recently dug up the cover to the tank and the tank is at the correct level. ANY HELP WOULD BE GREATLY APPRECIATED!!!!!

Sewer Vent Problem

Very rarely is a venting issue the cause of a plumbing problem unless there isn't one, (a vent) to begin with.
I would lean towards a restriction in the drain line downstream of the bathroom that has the bubbling toilet.