[He wanted to blurt out that he's not feeling well, but then Yamaguchi would start panicking even more clingy and demanding for him to stay here. But this is making him feel really, really weird. In a bad way.]

...I need fresh air.

[Fresh air to clear up his mind]

...My mind keeps getting random, ridiculous ideas as to why that guy was coming here, and none of them are good. I'll be back in a moment.

[He says that as he steps out of the room. He made a promise to himself he'd be more open with Yamaguchi and stop with all of the evasiveness. While he's not good at it YET, he's trying.]