Ok ive never posted here before so bare with me

Ive been in an english class for over a year and this one girl continued to piss me off day after day  by saying "HAAHAHA you drop the weights  you're so weak". Ive told her that this annoys me because i have some self image issues from being younger and not liking how skinny and weak i was so i started to workout. As i said this continued for over a year and one day i had a particularly bad day and snapped she had said" HAHA I bet you drop weights that are only an 1/8th of a kilogram" she actually says this so i said back " i might fo that but at least i dont weigh 80". She stops annoying me and i think this is done. Later on she gets her friend to message me calling me an asshole for what i said and that i should apologize i say no and explain why she then says" well she has self image issues too" but the girls i called fat isnt even fat i just said it because it made sense with what she said. I am still aparrently the asshole to these two and probably their friends and i dont see why i am. Seriously am i the ass

TLDR: Girl picks on me for a year and when i retort suddenly i am the ass for one little thing