Id be willing to bet it is some form of mold. I have the same thing happening in my bathroom on the ceiling because my wife and 7 year old son take super hot showers and the humidity level is more then the exhaust fan can exhaust. My plan is to cut out the ceiling dry wall and add largest cfm fan i can buy and then replace the drywall.
I suggest you have someone take a look at your problem so no one in your house gets sick.