Luna herself didn't really express that she was competing with Arit, so it was too subtle for Arit to point it out. So instead of beating around the bush, Arit decided that she would just ask the most important question. Luna's answer here would determine whether or not Arit could be friends with her.
Luna turned away from Arit, and at that moment, she allowed her nervousness to get the better of her. Normally, in a situation like this, Luna would be bold about her desires and she would come out and say exactly what she wanted. Luna knew that she liked Mark, and even though she wouldn't call it love yet, she knew that it would get there with time. But the way she was put on the spot like this, as well as the fact that it was Mark's girlfriend asking her the question, made her nervous.