Can I spend the night at my best friend's house? I'm a girl.

Of course. As long as they are good friends, regardless of gender, friendship exists. Don't think about anything, it will lead to stress and bad thoughts. Don't care about other people's opinions, just be yourself, and don't be afraid of the shadow. But at the same time, I also want to suggest that you pay attention to the details, and don't dress too exposed, which will cause embarrassment and is not conducive to the continuation of friendship. When the buddies meet, they can talk more about things and anecdotes in life, which will make the relationship between the buddies more iron.