@RooRoo, yeah, it's pretty much always been like that. Except before, she wouldn't try the doorknob and I would be drawing in a breath to answer when she knocks a second time. I used to hate that, but I think I prefer it over her just walking in.
I did have a talk with my mother about it, and it went very well. She said she would try to remember in the future, and that she just wasn't thinking. I think she was probably used to having access to me whenever she felt like it, because I was on the living room couch for a while.
Little side-story as to why I was out there for so long: the roof leaked right on my bed two years ago, completely ruining it, and in the interim between that and when it was fixed, my room was designated an impromptu storage space. After the roof was fixed, my room was still full of everyone's junk. It was a big cleaning job, and things kept coming up to interrupt it. We finally finished with that and replaced the bed a little less than two weeks ago. So, maybe her new habit stems from her being unused to my being behind a closed door. All I know is, I've only just gotten some measure of true privacy back, and I'd like it to be real privacy that I have some say in.
Anyway, we were able to discuss it like adults, with limited results. She already forgot once, caught herself doing it when she found the door locked, and said, "Oh, sorry. Can I talk to you, please?" So...I think she is trying to break the habit, and she just hadn't been thinking.