The last time you painted your ceiling did you notice spots around the outside walls? Those spots are usually an indication that the insulation in the attic is starting to loose its R value, and condensation may be the source of your problem.
Attic Insulation and Year Round Condensation
During the summer and winter the poorly insulated attics will accumulate condensation. In the summer this moisture can leak into the lower level ceilings of the home once it soaks through the insulation, and it could also become a source of mold growth. Once wet, insulation is useless and no longer carries an r-value. In the winter this moisture becomes frost in your attic that further diminishes the effectiveness of what little insulation you may have up there. This situation can be corrected by adding blown in insulation to your attic. This blown in insulation will help to minimize future spots from occurring on your ceiling, but more importantly it will help cut your heating and cooling bills. » Read more: Spots on the Ceiling, Moisture in the Attic