i said it earlier, but nevermind ^^"
i'm not 100% sure, but it was said that the ohagi was filled with tabasco (or another spicy sauce), which mislead keiichi. (under his extreme paranoia, he sensed a wound, instead of the spicy taste... duh XD)
mion didn't denied that she set a trap when keiichi was yelling at her, but she wouldn't understand why he was saying "blood" etc. (rena didn't know anything about the "wrong ohagi" )
EDIT :
i found my source! XD (in lostblue spoiler,
Tsumihoroboshi Hen )