Ok, if someone told you NOT to use putty on the fiberglass, and you didn't put anything in there, then i'd bet it IS your drain that is leaking. Go ahead and use putty on it, and afterwards, slap whoever told you not to.
isnt there subflooring above my ceiling? i really dont want to cut into all that if i dont have to .. i was wondering if i cut a acess panel in the closet to have a look at the faucet and shower pipes to see if there leaking? also the leak must be little because the spot was dry this morning.
My thought is that you will likely have to repair the ceiling downstairs. If you cut out the damaged area, you might be lucky and find that you have a plank subfloor instead if plywood. If so , then it would be easier to try and track the path of the leak to it's source. If as posted above, it's easier to cut open the closet wall to see under the tub then do that. If you can get hold of an inspection camera, that will help you. Once you have a visual under the tub, look for staining on the subfloor or mineral deposits on protruding bits on the tub. Maybe puff some flour or durabond powder under it and then run the water and the jets and see what happens.