Is your current post/question related to Leaking Bathroom post??? From the look and sound of this post, I'm beginning to wonder if the Toilet Gasket DIDN'T fix your 'past problem' ...
In any event, the problem could be the drain itself. IF you're able to seal off the drain, put some water in it. You don't need much because 'water pressure' doesn't sound like it's giving you the problem. (IF it was, you'd have water when NO ONE was in the shower.) If you're successful in sealing the drain off, let the water sit for a while. If nothing leaks, then you probably have your answer.
The only other cause could be that there's a leak somewhere between the faucet and where the water comes out the shower. There, you may need to replace the showerhead 'arm', or simply put some Plumber's Tape on the threads. I can't imagine that there's a hole in a piece of pipe somewheres but I guess it's remotely possible.
Try the drain test then re-tape the Showerhead Arm. It would be good if you can find a friend who's done some plumbing because you'll need some tools (for the arm), some tape (which goes on a certain way), and perhaps even a new arm (which if not 'padded/protected' from the monkey wrench during removal, will 'crumble' or 'twist' under duress.)
Assuming you're not really a DIYer when it comes to Plumbing, sometimes it's best to call a Pro. Doing it yourself could cost you MORE $$$ if you damage something in the process.