Bad odor in the KITCHEN when taking a shower in the BATHROOM

The first suspect would be a stuck/failed AAV if you have one gray tank. If you replace the AAV, I'd recommend the higher quality Oatey one that was posted above (photo below).

We had the AAV at our kitchen sink go bad in less than a year. Bought a replacement at Lowes and it didn't last long either. Went back to Lowes again and bought the better Oatey version and it has been fine. The AAV for our bathroom sink had to be replaced in under 2 years.