need more information.with what you've said already my guess would be if you have the type that has a knob on the tub stem that you pull up to start the shower it could be leaking where the spout attaches to the pipe especially if it's the kind that slips over the pipe and has an o ring to seal it.also check for voids in the tub caulking.check the back wall on the other side maybe there is an access panel you could take off to see whats going on.how long has this been doing this does it happen every time.could water be getting on the floor and leaking down?