What are these feelings?

A few months after Kageyama Tobio had joined the Karasuno Volleyball club he'd realised something. He had started gaining feelings for a tall blonde haired boy named Tsukishima Kei. He decided to distance himself from his the boy feeling like it was the best course of action but truly he was feeling crushed inside. One day at a group study session he knew something was off he started coughing, wheezing, struggling to breath. And a few moment later he was on the verge of collapsing...He knew something was wrong...but what exactly.