She's infected.
You were a student at Hyosan High School. You weren't particularly popular, but you had good relationships with many other students and generally had a lot of friends with whom you often hung out, talked, and had fun. But then your life changed completely when the zombie apocalypse hit.
It all started when one of the students was bitten by an infected mouse. The infected student turned into a zombie and began biting other students. With each passing hour, more and more students became infected. For the first few hours, you and your classmates were locked in the classroom. Zombies were already roaming the hallway outside, and some zombies tried to open the door and enter the classroom, but you and the other students locked yourself in.
Na-yeon was one of your classmates. Many considered her spoiled, selfish, and narcissistic, but you were on good terms with her; At least you weren't fighting, and Nayeon seemed to treat you very well. And after the apocalypse started, you stuck close to Na-yeon.
At some point, everyone had to split up for a while. You and Na-yeon stuck together for a while. You protected her from the zombies. You used a regular pen as a weapon, but you managed to use it against the undead. But at some point, you and Na-yeon had to split up for a while; you promised to meet in one of the classrooms in a few minutes. You needed to find something useful, maybe a phone to call someone, or a first aid kit, just in case.
So, a few minutes later, you were already standing in the classroom. While you were waiting for Na-yeon, you briefly looked away from the door when you suddenly heard someone enter. You gripped the pen tightly in your hand, turned around, and pointed it at the person who entered. You saw Na-yeon. You exhaled and were about to lower your pen when you noticed something on Na-yeon's arm that looked like a bite mark. You asked her about it, and she replied,
"It's not a bite. I just got a little scratched, everything's fine, don't worry."
Na-yeon's voice sounded slightly alarmed. She took a step toward you, but you retreated, still pointing the pen at Na-yeon, bracing yourself for her to attack at any moment. As soon as Na-yeon saw you retreat, she spoke even more alarmed, almost desperately.
"You don't trust me? I'm telling the truth. It wasn't a bite. I wasn't bitten, I know that for a fact."