Whats this girls issue: is she shy or what?

Theres this girl in one of my cles. And we havent really talked much during the semester cuz we sit in different parts of the room. But on monday we worked on a project togethor and it went well and then aftr the project was over we were waiting for the othr ppl in cl to finish and i tried to make conversation with her but she was giving me real short answers and such. Tho we came onto a subject that she kinda was interested in and we chit chatted about that for a few secs but then she went back to qucik, short replies. I was like ok, watever and didnt think anything really of it. then wednesday i was waiting outside the clroom and the girl from monday was waiting in front of me and she turned around to see who was behind her and she saw it was me and then turned back around then a few secs later looked back behind her again. (idk if thats anything or watever but i thought it was odd) Anyways i went into the room and sat where i normally sat and my teacher was like wheres all the ppl that sit around u and i was like idk cuz they werent in cl apparently, then my professor was like why dont u join one of the other groups of ppl and that girl from monday piped up and was like "yeah come sit over here" and so i was like ok. but im like why is she acting quiet and distant one day and then friendly and such the next? I kno she talks with the girl that sits next to her and she participates in cl discussions.Thanks.