But she never said she was walking and heard her talking about her. Talking behind ones back can conclude to many things including hearing from someone else saying someone is talking behind your back. Best to see what Cube or Soojin says about this.
"The individual who spoke of me always mocked and cursed at me for 2 years with her 'group' on the way to school, during breaks in the hallways, in the cafeteria, everyday, everywhere, saying, 'She's not even pretty so how is she a celebrity?', 'Her fame is already a thing of the past', 'No wonder she's an outcast', 'I don't know why the teachers always treat her so specially', repeatedly harassing me verbally. Sure, it could just have been ignorant youth and jealousy, and they could have uttered those words without any second thought. But those words are still deeply ingrained in my heart as scars until today. The scars that accumulated back then have become a large bruise, and I eventually grew to fear people. Those fears then grew into trauma, changing me into an introvert and hindering my advancement into high school."
Talking loud enough for the "victim" to hear without talking directly to them can be considered as "talking behind my back".