I would hazard to guess probably an "outside" pollution.... that is, something outside of the tank that polluted the air and now causing havoc to the tank.
This could be anything from sprays (bug spray, perfume, cologne, or???), smoke (possible e-cigs, cigarettes, diffusers, etc), heating something up in the microwave (styofoam container, wrapper, or?) to something else.
If it was an issue with the water you should have seen something occur sooner.
So consider doing a water change sooner rather than later.