Why is she so quiet?

My friend group includes this girl who seems to have little issue talking with other, yet she was more quiet around me. I chalked it up to the fact that I didn't really talk to her, so when I had the same class, I tried talking to her more there. She slowly became more willing to talk to me afterwards. It got to the point where I asked her how her vacation trip went, and told me that she realized she accidently left her passport at home when she was already at the airport. However, last time I saw her and decided to sit next to her to talk to her, she responded to me with short answers and generally just quietly stayed on her phone. I always have to initiate conversation, likely because she's so introverted, but is there something else going on? I mainly want to know if I just make her uncomfortable or if it's something else.

Why is she so quiet?
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