I can't tell what this girl is trying to say?

Im 14 and bi female who goes to an all girl high school. Theres this girl who is really mean to me, the thing is recently she’s been nice. She held the door open for me even though it took me like 3 minutes to get to the door, and she volunteered to walk to the tech lab with me. I don’t even know if she’s “not straight”. I don’t know much about her accept her name. How can i tell if she has a crush on me? and what are the signs that another girl does like me?