[Marc Lucas]"The side it happened on was the side the back light was on so yes something to do with how the light is interacting with the footage and green screen when trying to key"
That makes sense.
The way sharpening filters generally work is by increasing contrast along edges. With your camera's edge enhancement filter on it will see a bright white edge on your talent and say to itself "Aha! I should put a dark edge just on the other side of that light edge! My master shall be so pleased..."
Silly camera. Bottom line is that checking to make sure any sharpening filters are turned off in the camera before shooting green screen is essential.
You might be able to rescue this with some extremely clever blend modes/channel shifting/grading/roto or with the decidedly less clever Refine Matte effect. I've used it and it works really well sometimes, but if it doesn't you've got to move on to other options.