Maybe it's something that's always been in the back of her mind..? 134 really makes it seem like.. Fumi thinks Shinobu isn't willing to let her do the scene, afraid that she's also gonna be "stolen away" like Sakura, and that might have made her think that Shinobu views her as "weaker" than Sakura, to the point where he doesn't even wanna take that risk.
But this is some gear 5 levels of stretching. No sane person would think about it like this. Even if they did, they'd talk.
Every single thing, from Sakura's betrayal to now Fumi's apparent betrayal could've been avoided if they just opened their mouths and talked like actual adults. But no! We need to emphasize the "NTR" in NTR comedy..
Smh.