What the heck is this stain by my window?
  Rookeh @   Rookeh @startrek.website       Posts 0Comments 81Joined 2 yr. ago 
 Rookeh @   Rookeh @startrek.website     
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As others have said, 100% a leak.
I would advise to stand on a chair or stepladder underneath the ceiling and check to see if it is still level. If you see an obvious deformation around the stain, this will be being caused by water pooling on top of the ceiling plasterboard. In which case, once the leak is sorted, you will likely need to drain the pooled water, cut out the damaged section, replace it, then replaster and repaint.
We had exactly the same issue in our last house. It was in a difficult to see spot hidden behind our kitchen cabinets. We only realised the severity of the issue when the ceiling boards gave way and fell on my head.