A similar thing happened with my upstairs bedroom - the water tank in the loft overflowed causing a drip which built up above the ceiling. I noticed a dripping noise on the carpet and after a couple of minutes of ignoring it noticed that the ceiling had drooped and had a hole in the middle of the distortion. We had to call a plumber and poked a larger hole in the sagging ceiling causing it to drip into a bucket faster and afterwards got the whole ceiling replaced.