Why would guy that tutors me seem worried about me being friendly with a girl he works with?

I gently flirted with him just one time about a month ago on text so my brain is in overdrive thinking it’s because he’s told her about me.

she came over and greeted him then said “hi (my name)” and he looked very worried and shocked and looked at me with a concerned expression on his face as if how are you guys on first name terms. Ps this woman is very attractive so I’m not sure if perhaps he is into her, he has a girlfriend though.

Why would guy that tutors me seem worried about me being friendly with a girl he works with?
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