Are the shower and bath one unit ? I ask that cause you mentioned a shower curtain but also said it occurs about the same time as a bath.
If so then the leak if it is from this one unit would happen anytime the unit is used.
The bath tub controls and drain are normally located on the same end, this often backs up to a closet in a bedroom, or a bedroom wall, You can buy at most hardware stores "inspection panels" where you cut a hole in the wall (to match the panel size) then caulk in the panel, this panel has an interior section that can be removed and replaced any time you desire and can be painted to match wall colors.
Before you cut into the ceiling I'd get myself a panel and then cut in behind the tub on the wall in the bedroom, this will enable you to see the complete drain line behind and under the tub and in most cases enable you to make the repairs from this area and then install the panel cover. Much easier than dry walling etc...
Panels are available in the plumbing section of Lowes/HD.
Are you sure the leak is not from the toilet ?, it is common to hear about a small leak in a toilet that once "repaired" (not by a plumber or experienced person) the leak gets bigger, Wax seals can be tricky things to install. I also wonder if this is the problem area because you mentioned a new floor, if it was "tiled" last year did they install a "extension ring" on the toilet flange before they installed a new wax ring?
The toilet flange (big plastic PVC fitting, screwed to the floor, that the toilet bolts to) should sit on top of any finished flooring, I often see people just tile up to the ring and then throw on a wax ring , problem is the flange is now at least 1/2" lower than it should be. Of course if you had vinyl laid on the floor there would be no need for the extension ring.