When Your Pain Becomes Mine

When Your Pain Becomes Mine,

A man who feels the pain of his lover after a blood transfusion :

The man (GyunHee), with a voice colder than the breath of winter, speaks to the woman(Sunny).
"Wait until I contact you again."
But his car slips upon the wet road not long afterwards and crashes
"Please save him! Please!!"
Sunny shouts desperately for her boyfriend' life as she arrives at the hospital.
"What is your blood type?" The doctor asks.
"...Pardon?"
He was in dire need for a blood transfusion, it was a common question but she could not answer.
After giving her blood to save her man, she decides their relationship wasn' one of love and decides to leave him.
And as GyunHee wakes up a few days later...
He decides to chase after Sunny..
But what is this pain that he is feeling?