Hello ! Can't remember the name of this one shoujo-ai manga. In last chapter what I red about 1-2 months ago was sports festival(?) or something like that going on and the other ones ex-girlfriend is there and says to other something along the lines: "Keep away from her she is mine." at the end of the chapter. Can't really remember anything else except that one line from the chapters end. I know it's hard without anymore clues, but I belive in you guys.
And I think the ex-girlfriend was her senpai.

Sounds like Asagao to Kase-san.

Yeah, just that ! Thanks a lot!