Help,How do I fix dirty water which comes up in downstairs shower?I think water is coming from upstairs kitchen sink.This used to happen when washer discharged but I put washer on different line.Any help would be appreciated.

This means, your common drain is clogged and needs to be snaked. Do you have any clean outs on the bottom floor and in the vicinity of the shower ?

Milo nailed it! You have a blockage downstream from the shower. Not all hones have cleanouts, especially those houses on a slab. If you don't have a cleanout you will have to snake from the kitchen roof vent. Put our enough snake to reach the base and 20 feet more. Good luck, tom