You can review my answer to Sandy's Post below for some insight. Also, it's possible that the leaking water is 'traveling' along a piece of pipe or joist or whatever, over towards the wall, and down it.
Try my seal-off-the-drain idea and try and determine if the leak is originating where the shower drain meets the drain pipe. You see,, you didn't say if you have a fiberglass shower/bath or what so it's possible, also, that you you might have a pin leak in the shower/bath itself.
Unfortunately, these types of leaks are difficult at best to pinpoint. If you end up calling a Plumber, he'll probably have to rip out ceilings and walls and perhaps floors too in order to find the source of the leak. You can do this yourself having the advantage of 'controlling the inevitable mess' you make. And looking ahead, once you find the leak, do you think you can fix it? (Kind of a rhetorical question. No need to answer.) Once you do find it, you may want to call a Plumber to do the actual fix. Of course, when the fix is all said-and-done, DON'T haste to put things back together until you're SURE you don't have any more leaks. It's quite possible, you know, that 'loosening' something in on place may cause a leak elsewhere. That is all.
My best to ya and hope this helps.
Jay J -Moderator
PS: If you have drywall, be sure to cut from stud-to-stud. This way, the repair is easier to do ...