Flush the toilet and I hear gurgling in my shower
it means that your shower is plumbed up with an absent vent line or a vent that is blocked or plugged.  if it just showed up recently, i would look for an ice cap on a roof vent pipe.  if its an ongoing thing, then i would suspect an absent vent pipe.  you can quick fix it with a rubber drain cover.

http://www.amazon.com/Do-it-Rubber-Sink-Stopper/dp/B000DZD3QA/ref=sr_1_1?s=home-garden&amp;ie=UTF8&amp;qid=1457233324&amp;sr=1-1&amp;keywords=rubber+drain+cover