I don’t think it’s the short hair. My wife and a couple of her sisters have short hair. An example of even shorter hair is Sinade O’Connor, when she was younger: http://cdn.mos.musicradar.com/images/features/cover-versions/sinead-o-connor-corbis-630-80.jpg, or http://images6.fanpop.com/image/photos/35200000/Sin-ad-O-Connor-sinead-oconnor-35217109-1000-1001.jpg
I think the short hair removes a visual cue. Her clothing could also be male or female. I think the third thing is your lighting, which is quite hard in all four photos. Fourth, is her poses of which two are square to the camera. Usually women are turned a bit to give them some shape. Petite women are turned a little and rotation is increased along with weight. The first pose is a masculine stance, near shoulder down. The third is profile and the jacket is fairly shapeless.