Damp patch in new plaster?

I bought a 1950s house last yr to renovate. All walls skimmed, including bedroom which was papered, before xmas. Then painted - mist coat & then emulsion. No probs till today. Small triangular damp patch now appeared in window recess in bedroom. It's where window, window sill & wall meet. Stood outside in heavy rain, no leaking gutter.How can I cure it please. Thank you.