Gap around interior window

Hi, All: I have a window in our upstairs bedroom that has developed about a 3/16 inch gap between the sill and the window. The gap runs the length of the window (about 8 feet) and extends up both sides. The house is only six years old, and the gap just showed up this spring. My first question is what might cause this gap, is it a symptom of another unseen problem? My second question is what is the best stuff to fill it with? I can feel cold air coming through it. The window is vinyl framed. On the bottom the sill is wood, the textured drywall comes right up to the window frame on the sides. Thanks for any input.
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