Apologize for your mistake and assure her that you’ve learned not to share your work, even with friends you trust. Although it’s not the case here, sometimes ideas even get borrowed accidentally (they read it, then later have an idea that they forget they saw in your paper). You never know.

What is kind of troubling about what you say is the part where you say, “but if you do copy, change it up.” Can’t tell from your statement to what extent you’re aware of this, but plagiarism isn’t just in cases where entire sentences and paragraphs are copied word for word. “Changing it up” with the help of a thesaurus and some rearranging is still plagiarism! Any material that’s not your own ideas–whether words, not-so-commonly known facts, or paraphrased ideas–can’t be used unless they are properly cited, and even then most of the paper should be your thoughts, not cited material. Rewording, translating, rearranging, summarizing, etc. are not enough to safeguard you from plagiarism. Of course citing a friend’s paper as a source is a little strange, so you’re best off avoiding reading each other’s work at least until you’ve all actually finished writing.

You might also read your school’s honor code about plagiarism. Tell your professor you’re doing so, and ask her a few questions about anything in there that you don’t understand or situations where you’re not sure if it constitutes plagiarism or not. It’s something you need to know to avoid future problems, and your professor will be glad to see that you are taking steps to inform yourself so it doesn’t happen again.