I'm not too sure how to find this leak. I took a shower this morning for about 20 minutes and when finished my bathroom was filled with water about half a inch high. The thing is I have two bathrooms and when they took a bath in that bathroom it also was filled with water. Just wondering where I need to look because my sink is out side of the restroom and when I turn the water on nothing leaks. Also the water while a take a shower is a little slow to go down but the shower did not over flow either.
i advise you contact a licensed plumber to look into this issue you may have a blocked sewer. try flushing the toilets and see if they drain like someone just put a whole roll of tp down it. also watch the shower drains as they flush if water rises in the drain you have a problem and also check the basement if you have one you may discover it is full of sewage.
Is there a floor drain in the bathroom other than the shower? Was your washing machine running while you were showering? I ask these questions because some times when the washer drains with additional (shower) water it can back up at the lowest point. If that is the case the line needs to have a snake run down as it will probably get worse over time.