Same thing happened to the ceiling in the shower at my previous residence. The water from a leak above was, as Matt A. suggests, held by a layer of latex paint. How paint could have the tensile strength to hold that volume of water without rupturing, I can't say. I was shocked to find myself standing there starkers, looking at just this kind of enormous, surreal bulge from above. Oh, and Matt, a friend and I did indeed hold a large bucket beneath it and sliced the end with a box cutter. Whoosh!