what is the reason that my friend is mad at me?

what is the reason that my friend is mad at me?
So im going to explain in details becuase i really need help to figure out the reason my friend is upset with me. So what happen is that my friend and i are in the same maths class and when it comes to test, sometimes we study together. Because there was this one time when we studied together we got really good results, especially because she needs some extra help from me. But i often reject her due to my work schedule. Anyway, so today one of my friend told her i don't have work on the weekend and so she ask me if we can study together for the upcoming test. However that was not in my plan although i'm just going to study at home. But i didn't want to study together and i wanted to work in my own pace. So i told her no i didn't plan it, i don't want to. And she got upset with me... and so i told her she can call me if she need some help but she was clearly angry and told me she doesn't care anymore...
like why is she so mad about it?