It's possible that Sachiko left Hiroshi alone after the fire. It's possible that he heard the nurses talking about seeing the ghost of the old woman who'd died that morning, and his stressed state, coupled with any medicine they gave him, may have made him hallucinate Sachiko was stalking him in the hospital. Note how in chapter 9 he says that he's "somehow tied to the bed", but is perfectly able to move and get up after it. Hiroshi convinces himself that Sachiko is there, calls his friend babbling about it, then hallucinates seeing Sanae with no face in the hallways and that the hospital is empty. It could be that he was actually being chased by medical personnel, which he thought were Sachiko, and that injection was merely to subdue him.
Chapter 10 (where this all goes down) is called "Paralysis", which I initially assumed meant he was actually in bed hallucinating all this, but he couldn't have called his friend like that. Therefore, it probably has to do with him feeling helpless against Sachiko, who may or may not have been there at all.
It's also possible for Sachiko, if she were a supernatural entity, to make Hiroshi hallucinate all the events in the hospital that night.
Considering Yamamoto's reappearance in chapter 11, it could be that Sachiko had found herself a new target in Hiroshi, and when he went insane she was either bored of him or achieved her goal. Then when Yamamoto, "the one who got away", came back asking questions, she appeared in front of him again. In the end, both Hiroshi and Yamamoto became as obsessed with her as she was with them.
ETA: and in this panel, I think Yamamoto is the one answering the door (note the short, dark hair; Yamamoto is the one narrating):
followed by another panel showing Hiroshi doing the same on one of the next pages (the narration is done by a few girls walking to school, one of which is Rumi):
So it's possible that Sachiko was trying all the apartments in the complex?