Ceiling Damp Spreading the more I damp seal it!

[andypaulcox]

Just a small spill upstairs which got under waterproof flooring seems to have caused a big damp patch on the plasterboard ceiling below. It wasn't dripping so it wasn't noticed for a while, so I guess it couldn't evaporate upwards due to the waterproof floor - which has now been removed. After a month I assumed it had dried so painted stain patch on the ceiling with damp seal. The next day the stain had expanded beyond the paint. I let dry for another month, but the same happened! The patch gets bigger the more I paint it with damp seal! Help what do I do?