It's not the hair style. In some photos you can see her collarbones underneath the blouse and that wool overcoat is just hanging off her. She is too thin. This is the photo that startled me. It's not the angle playing tricks on the eye. You can look at photos of her from even six months ago and you can see the difference.

https://www.gettyimages.co.uk/detail/news-photo/catherine-princess-of-wales-during-her-visit-to-the-news-photo/2297310843?adppopup=true