You mean behind my back?

他们背着我说我坏话了？

重点表达

behind one’s back：

Meaning to do something without someone’s knowledge, secret from someone

背着某人说坏话，做坏事

例句

How can he go behind my back like that! I didn’t even blink my eyes when I lent him 10000 rmb!

剧本台词

Monica: Phoebe, do you think that your favorite animal says much about you?

Phoebe: What? You mean behind my back?