The drywall man is good at what he does, I'm sure, but you need either a plumber or a tile setter to diagnose the problem. You first need to do a shower pan test. The "bladder" you're talking about is properly called a "shower pan," even though it doesn't look like a pan and you can't see it.

I've posted a little article you might be interested in:

• Shower Pans for Tile Showers