@Xazer
MC is jealous of Hiyama (lesbo)'s appearance and popularity, because MC has very low confidence in herself and that she wouldn't be a good match for the cool looking/popular guy.
MC's low self-esteem is annoying, and some people would think it's illogical, but lots of teenagers do have similar issues.
Translator adds that kind of note but you could also say that in a "comedy" that relies heavily on female-on-male disproportionate violence and get the same non-reaction. [Shrug]
Pretty sure these are Flashbacks. Not that she has amnesia but that the chapters are generally out of order...at least, that's how I get at it with the order that the Serialised Version goes for