My teacher keeps looking at me?

Its really weird, like he glares at me and when our eyes catch sometimes he looks away and I can feel his eyes on me when i look away and see him looking at the corner of my eye. He looks at me when I p him on corridors too but only says hello when I say it. I'm not just being paranoid because my friend noticed too that he looks at me funny but she thinks he dislikes me. I don't know because he's really nice and teases me jokingly and always asking if I know what I'm doing. It so weird and makes me feel awkward! Whats his problem?