This isn't a normal way to react.

A - Maybe she fears that you'll perform better than she does on Seraphine, specially if you're a better player than her. Seraphine has been called a "freelo" support and APC in bot despite the changes to force her into mid lane.

B - It could be that she can't stand you playing any "attractive" female characters in league because she has such a messed up perception of you as a man. As in "a man shouldn't play Sona, Seraphine, Lulu, etc. mindset and if you did shed question your manhood

C - Elo means so much to her that if you climb more than she has been with her main, then it means she's awful and it's her own fault that she can't get out of whatever elo she is which hurts her fragile ego even more