This was taken at 1/500sec, iso64, f/2.8, 122mm focal length. Nikon D850, 70-200mm lens.
At first I thought it was her hair, but it seems to be very reflective and moving very fast:
It even seems to change direction at the bottom. If it is at the subject's distance, that's moving, say, a foot in 1/500s. Or over 300mph. It's seems out of focus, so probably closer to the camera. It seems to change speed since the brightness of the streak changes?
Is this not a streak and just a fiber on the sensor? I did not see this in any other photos.