Because Carrow was horrid to her after that article about her father came out.

Actually I just went back and read in her journal that entry from earlier in the month. Mr Malfoy told Carrow to watch his step and Carrow kept right on yapping.

This is the first time this month Mr Malfoy's been to the school, unless I'm forgetting something. Awfully convenient Carrow was in the hospital wing and Mr Malfoy couldn't see him I imagine Mr Malfoy thought so, too.