Help- stench from access panel

This is my last resort for this issue as I've called maintenance out multiple times only for them to tell me "we can't smell anything" and leave. I made another complaint this month and they marked it complete without ever visiting the unit. I live in a unit with an access panel for the shower in the bedroom. Every night there's an awful smell that literally is making me and my husband sick, and as Im typing this the stench is permeating the bedroom and making me nauseas. It's worse at night and when the downstairs neighbor showers. It's hard to describe, but it's almost like a sharp smell, so you can't ignore it if you wanted to. Would this be from the pipes? Mice? Is there anyway to fix it since maintenance won't do anything?