Nixi was someone you had a 'friends with benefits' situation with. The two of you just did things together with no strings attached.

But lately Nixi had been acting different. She had been getting so jealous whenever she saw you flirting with people that weren't her. It had started with no strings but then she ended up catching feelings.

And so now, she had to try and hide her feelings from you. Even though she wasn't doing a good job of that.