I recall reading that she had almost died following a severe illness which has been described as anywhere from blood poising from an infected cut to typhoid. Her recovery was considered miraculous according to her father who wrote about summing father John of Kronstadt to her bedside when it was believed she would not survive. It's after this illness that she appears more slender, with dark circles under her eyes and yes, short hair. It was fairly common to cut off your hair after a serious illness back then, so I believe this photo was taken not too long after her illness.