3 year old Septic Tank full?

[mysticokra]

Check to see if the shower has a trap.
If not, that may be sewer gas you are smelling.

Part "B" to that check out would be whether your system is vented. Check to make sure the vent pipe is not clogged.

[StanleyinMd]

mysticokra hit on a good point about whether the vent is clogged. Check to exit point on the roof to make sure a bird didn't build a nest there to stop the flow. gurgling and smell could indicate a clogged vent.