??

Probably from the gunshot wound (ch.61)

It's probably because of their psychological bond (because he had pain in his side on the mall scene). Jae-In was stressed out and afraid and the doctor had told Jae-in around chapter 40-45 that the alpha was the one feeling the symptoms of their psychological bond. If they were to form a physical bond then Jae-In would feel anything Taeyoon would be feeling. Kinda cool, huh?