The reason that Dahyun was in a hurry it was a message from her mother.
 

"Dahyun come home, I need to talk to you"  

She was worried, she seldom write to her. As Dahyun hurriedly came, she saw her mother was in deep thought holding some files in her hand. Coming closer to her mom she only welcomed her with a warm smile on her face but Dahyun felt that there was something wrong and she was hiding it. 
"Sit down Dahyun. What happened to face?" Noticing the wound on her face, she gently caressed Dahyun's face.
"It was some guys that attacked girls so I just barked to them," Dahyun said hiding the truth about Sana.
"You are like your dad. How come he cannot be with us. Probably he is guiding you from above that you should do good things" the voice was sad in Ms.Kim.
"Mom, what's wrong?" Dahyun worriedly asked.
"Can you check it ?" Ms.Kim gave documents including proves and medical records and references that she has cancer of blood Leukemia. 

Hospital---------- Seoul year ago Dahyun's mother finds out that she has cancer of the blood. 
She couldn't say to Dahyun since she was far away from her and she was sure that she will assume that return because of her. (dropping from university). So, she kept silent and didn't say anything. She decided that she would say when Dahyun will come back home from London. Having Leukemia the cancer originates in the blood and bone marrow. It interferes with the bone marrow's ability to make blood cells and plates. For 2 years the cancer has been developing gradually but she took medical treatment without saying Dahyun. The doctor said that she needs chemical treatment in the hospital. She wanted to say about it because she has a little time. 

After reading and checking all records and references only tears appeared in Dahyun's eyes. She couldn't believe it. 
"Mom..... How could you suffer alone? You should have told me a little bit earlier mom." - the worry and pain slowly ate her.
"I couldn't tell you because you were in London. I was sure you will quite from university just to be here with me" her mother said still keeping calm
"You will be fine mom. I will be with you for sure. We can make it mom... "  Dahyun always think about positive sides whenever something happens. She hardly accept the truth that was worry for Ms.Kim so she just agreed.
"I will start my medical treatment from December Dahyun-ah and I will be in hospital.  " her mother informed her.
"I will beside you mom.. don't worry, " Dahyun said smiling that everything would be okay.
"Don't focus on me too much. Don't forget your studying. You should promise that you won't skip your classes and preparation for a huge project that will be in spring since I want to see how you will play for thousands of people in Grate Hall of Seoul. It is my dream to see your great performance in the Great Hall of Seoul. Promise me that I will see your performance Kim Dahyun and don't forget to smile whenever you will go" - Ms.Kim said with an assured tone.
"I promise mom" Dahyun smiled even there were tears. She was afraid that she can lose her mom.
and mother and daughter made a pinky promise 
_________________________

Accepting the truth was hard for Dahyun. She only focused on treatment and positive points. For a while, she couldn't think about Sana but she cannot say it tho to her.
The next day Dahyun is standing in front of the door of Sana's dorm. Important to emphasize who knew that Dahyun was really good at hiding her feelings and emotions only showing a sweet smile. 
After opening the door Sana immediately welcomed Dahyun with a warm embrace without knowing that Dahyun was carrying worry and pain inside. Dahyun could find warmth and comfort in a hug forgetting for some time that her mom is ill. 

"Are you hungry?" Sana asked Dahyun who just sat on the sofa.
"No. How was your day? " Dahyun asked Sana remembering the boy Leeming.
"Actually, he was captured by the police at the club. Today students were talking about him. You were right he was under drugs yesterday and knock down some people there  " Sana said sitting next to Dahyun giving some juice to relax.
"Should I be happy that you are finally safe" Dahyun said before drinking the juice.
"I have news for you," Dahyun said putting the glass on the coffee table. As Dahyun turned to Sana. "I will perform in Great Hall in Seoul in spring. We are getting ready for rehearsals" the happiness and at the same time there was nervousness in Dahyun's voice. As Sana gently grabbed her hands "You can do it" she said with a reassuring smile and give a peck since she was craving for long. 

"I think your kisses are the best things I have ever had" Dahyun admitted closing her eyes until another sweet kiss landed. All hugs and kisses of Sana took away Dahyun from a world of worry. 
 

"You will be the best part of my life Dahyun-ah" Sana said cuddling Dahyun on the sofa. 
They spend the evening cuddling with some kisses and funny stories. Both of them love it. Especially Dahyun. Such routine continued until November then there was start  Dahyun's rehearsals for the project. After classes, the first thing Dahyun does it is calling to her mother to make sure she is fine. Then calling to Sana that she would be there after 30 minutes in her dorm. After 8 pm she leaves Sana to go to evening rehearsals then she will go home hugging her mom from the back, assuring her every day that everything will be okay. Spending times with Sana sometimes end with interruption from Momo in the middle of the make-out session.  Smiles and sparky eyes always remained but Sana sometimes notice a little tear when Dahyun sleeps. She noticed that she is currently facing a hard time but she just waited for Dahyun to speak up. There would be always time for it.