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She may have become sensitive in response to something that may have taken place about 2 weeks ago such as witnessing something outside or on TV that had taken place outdoors.
She may also be doing this without any external cause for it.
It may be possible to engage her in age related play and try to extract what the reason is (if any) or you could also engage her in the play or tell her a story that takes place in a little girl's room with the windows open and has a happy plot and a happy ending.
You could also try to have one of the curtains open and let her see how nice the light reflects into the room and gradually day after day open the curtains more.