Yellow stains on the ceiling, how to diagnose
The first step is to get up into the loft (assuming the bedroom is on the top floor of the house) and look for signs of water up there. You should have insulation of some sort, this will hold the water. 

Then check for nearby damage inside and out. Binoculars can help with the latter. You'll probably have to get it professionally repaired but you need to stop things getting worse until that happens. And you may want to get in touch with your insurer.