This sounds like an acrylic tub that wasn't placed in a setting bed. If you can get access to the underneath, you can try shooting in an expanding foam to add a little support under the tub.
Stay out of the tub for the curing time stated on the label.
There were posts in one of the categories in this forum within the last two months on this subject, along with some solutions as to how to fix the problem without tearing out a wall or floor. Do a search to find these posts. Thanks, David