Girls, I called her beautiful and she told me to stop?

I have called my (single) friend beautiful twice now, and she has told me I need to stop because I apparently don't know beautiful.

Is this her setting boundaries, or is it her insecurities and not believing she is beautiful?

Setting Boundries
Insecurities
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Should have included this withing the question, she has a very harsh view of herself and puts herself down a lot, both on her achievements and her appearance.
Girls, I called her beautiful and she told me to stop?
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