First, because it's a brutal change for Mask who had established a little while ago that she was happy being friends with cute nicknames and everything.
Nah, Mask knows very, very well that Kaoru is attracted to her. She knows doing that cutesy stuff will encourage her, which is why she does it.
Second, because it doesn't make sense. If she felt a strong attraction to Kaoru the moment she saw her it just doesn't make sense the way she responded to her advances.
Mask is blushing like crazy when Kaoru first starts hitting on her. She turns Kaoru down because despite being attracted to her, she doesn't want a relationship at that time, and probably knows about her being a player. She starts acting colder when Kaoru refuses to take "no" for an answer.