What does it mean when a girl is worried I'm mad at her?

This girl and I have been very close recently like we talk to each other all most everyday. She is very chatty and stuff. We have hung out heaps as alone and in groups. Recently she has been really quiet, taking ages to response and replying with very short responses. I finally got her talking and she said that she was worried i was angry at her and I really actually wasn't. Suddenly she is now asking when I'm free next. She also said that there are other things been on her mind aswell. She is sending me kisses again in text message.
What does it mean when a girl is worried I'm mad at her?
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