Why is she like this?

A little over a year ago I met my best friend (I was 17 then), 6 months ago I told her I had feelings for her and she turned them down. This was the first time I've ever had any real feelings for anyone. After she turned them down it was the worst week of my life before we talked again and began reconnecting. We became closer unbelievably faster and for 6 months after that up to today we have one day every week (Sunday) where we go out and spend the entire day alone. We go to different schools and I live a 40 minute drive away from her. Within the last 6 months we went from spending that one Sunday every week together. As we continued to get closer we went from maybe 2-3 days a week to 3-4 days a week together. And in the past 2 months we've began spending 5-7 days a week together where after school I would drive to her school, pick her up and we would just go out, we would spend about 7 hours together and I would bring her home around 10 every ni ght. For my 18th birthday 3 months ago she got me the nicest present I have ever received from anyone outside my family. She held out to exactly 12 a day before midterm exams to send me a long text wishing me a happy birthday. About 2 months ago I asked her to my high school prom and she said yes. Though it was an quiet and not so excited yes, I actually did go all out on asking her by setting up balloons and signs and chocolate and confetti and everything. After that event is when transitioned from spending 3-4 days a week to 5-7 days. She began being far more playful. Lately we've been seeing each other every single day. We talk every night even after I get home from driving her home and would talk on the phone until one of us is too tired to go on. Every time we have nothing to do together we just sit in the backseat of my car where she lays on me with her head on shoulder while I hold her. We would stay like that for 3-5 hours at a time and feel perfectly comfortable. On the night of prom a week ago we left the after party at 12:30 and just went somewhere where we laid there and talked till 4 in the morning before I drove her home. Can this really be just a friendship because I'm more and more convinced she wants something more. But the haunting memory of what happened last time prevents me from ever saying anything. Can anyone explain to me why she has become this way, could it be that she changed her mind?

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