[Stripit]

Hard to tell, could be a water leak that got the floor soft, could be a support structure across the floor broke loose and is pushing upwards on the floor. If it is soft, and squishy or bouncy that means the floor is not secured to the substructure I would think. Until you/someone gets under there and looks, hard to give a defined answer. If structure broke loose, reweld back to where it needs to be. Flooring wet/rotten, pull carpet, cut out bad wood, install new and if done carefully, reinstall and stretch old carpet. Or install new carpet at that time. Either way, I would not let it go as it can't get better by itself.