Any painting done or photograph taken is naturally copyrighted by the artist, so she is definitely in the wrong to just inform you that she is using your image. Asking you is not just more polite, but necessary… If you feel uncomfortable, tell her that you’re just informing her that no, she’s not allowed to use it. Or if it is a good marketing avenue, tell her to credit you in her post.
I have come across some really interesting images on instagram, and asked the artists if I could use it for my art. They said yes with crediting. But in your case, I might have been so pissed off that I will just say no (she didn’t ask you, but inform you… what kind of attitude!)