Basically she fell in love with MC, and became assertive/won't give up that easily, even if he gives her the cold shoulder..
In a sense gotta give her props, for not giving up right at the start.
Also, I don't think her behavior-, regarding what happened after the duel, up till the chapter's end, -isn't a "yandere" behavior.
At least not yet, since I don't think she's done anything proper crazy/threatening/aggressive yet.
Basically she's just "marking" MC, in front of possible competition, or at least just showing it publicly, that she's interested of him (/being his woman).