Why is she so defensive?

I'm friends with a girl and we get along just fine. We hang out all the time and physical contact is often there, so there's not really any barriers between us and there is nothing romantic which we have previously discussed so we both know how it stands.

My question is in relation to her coming over to watch a movie last week. She brought over a takeaway after work and showed up ready to watch a movie in my college dorm room. Things were chill as usual however when the movie started I lay back on the bed and she stayed sitting in the corner, she eventually moved into the middle and then eventually lay back on the bed, however her arms and legs were both crossed and she stayed on the literal edge. I began to feel uncomfortable because I felt like she seemed uncomfortable or defensive, even though all I done was watch the movie and pass the odd remark, but didn't do anything out of the ordinary.

Once the movie ended she got up and left, not really saying anything. I didn't know how to feel because she didn't seem relaxed at all. Yet a few days later she asked when she could next xome over to watch the next movie? We hang out all the time anyway, we go to the cinema and we've often watched movies online together, so it's not like she has to come over, which really confuses me.

She seemed really unrelaxed and defensive throughout the movie, so why is she so keen on coming back again when she doesn't have to and seems a lot more at ease in every other thing we do? And in case anyone mentions it, she's the one that friendzoned me

Why is she so defensive?
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