Why she look so sad around me?

I was not sure which category this belongs to, so I apologize if it is in the wrong section. I have this friend who i know at my job. She rarely talks to me but she looks at me a lot for some reason. When i walk past her and she sees me, she does not say anything but rather she just bowed her head (sometimes) and looks down almost like she is sad or something. She does not cry and i dont recall being mean to her or rude. When she is with her other friends she just normal. But whenever i tryto talkto her, she quite often has this sad look to her. Anyone know why? I am never mean to her and i am also nice to her.
Why she look so sad around me?
Post Opinion