Help needed with backed up shower drain

We are having a plumbing problem and I'm not sure how to fix it.
What is happening is water is backing up into the master shower (the furthest point of the plumbing) and it has a horrible smell. We haven't used this shower in a month or so because of it.
Sometimes it happens once or twice a day. I thought it was a venting problem, but as far as I can tell it is clear.
Any ideas of what this could be?

Your drain is clogged and judging by the bad smell it's the main...

Call a plumber to get your main cleaned.
Don't do a load of wash right now...

What Redwood said plus. Don't try to clear this with chemicals. They won't work and they will create a hazard for the plumber who you will eventually have to call. Get a real plumber, one who does not have RR in his company name.