Is this happening to me?!

Ep. 1 One Day

One warm, sunny morning...

Tiring-tiring

태형

Hmm... I slept well. What time was it?

태형

?! 8:55?! Ouch! I'm late!

태형

This bastard left first without waking me up, right?

(Getting ready to go out)

태형

Ah.. If I'm late, I'll get in trouble with the teacher.. I have to hurry.

태형

Huk! Tongue... Hyung?

태형

Why are you here, hyung? Didn't you go?

남준

Hahaha today is the day I'm not going!

태형

Oh really? (I have to get over it as best I can, like nothing happened)

남준

Yeah... didn't you swear at me earlier?

태형

Me... Me?

남준

Yeah.. you, you work!

태형

Oh my gosh!!!!!!!!!!!!!

dump

남준

Taehyung? Kim Taehyung?!!!!!!

hospital

의사

What brings you here?

남준

He suddenly collapsed!

의사

Well then, let's do a test first.

An hour later...

의사

Yes, the test results show nothing abnormal.

남준

Yes? Then why did you collapse?

의사

I think it was because I lacked stamina.

남준

Yes, I see.

When I come home

태형

Hmm... where am I?

남준

Are you awake?

태형

Uh.. uh

남준

Are you okay now?

태형

Uh.. ah..

남준

Then you should pay me back for cursing at you earlier, right?

태형

Huh? Huh! No, jump out!!!!

What exactly is Taehyung's fate and why did he collapse?