Did you install new floors?

Hard to diagnose. The problem is a trap seal: lavatory, shower, or toilet. Or..your wax ring is not sealing.

Alot of times when a flooring install is done, the flange is not set to the correct height, causing a very large gap to be filled with wax.

Is it a new shower insatll??

If it is just a toilet replacement and nothing else, that really narrows it down to the wax ring. Did you use a deep set seal? or possibly a jumbo wax ring? That may help.