What‘s a good answer to „Why are you shy“?

I went to a birthday party yesterday. There were 6 people, two of them were my friends and it was the first time I met the other 3 which is why I was a little shy. One of the other girls asked me „Why are you so shy?“ And it really hurt my feelings and made me feel uncomfortable. I didn‘t know what to say. Do you think it‘s rude to ask people that question? What are you supposed to answer?
What‘s a good answer to „Why are you shy“?
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