Guy's Behavior

Why he utters "bitch" in his breath whenever he sees me, but stare at me?

This guy is my classmate and I am pretty sure that I have not done anything wrong to him at all. Recently, everytime he coincidentally sees me, he utters "bitch" under his breath when there is no one except for me and him in the space. But the utters is soft, but I happened to hear this. But one time he said this, as I was walking back to my room, and with my peripheral vision, he kept looking/staring at me while trying to hide behind his door. This didn't happen the first time too, before, I was just standing in front of my room and was looking at the right corridor. I turned my body to the left swiftly and looked left , only to see that he was staring at me the whole time while hiding his body behind his door (his head was peaking out) and it was just us there. Then he quickly looked to the other side as if he was checking something then quickly went inside his room. I am pretty scared to ask him about it tho.

Why he utters "bitch" in his breath whenever he sees me, but stare at me?
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