Do your MIL let herself into your house?

My MIL has a habit of letting herself into our house. Sometimes she knocks, others she doesn’t. Sometimes she comes unannounced and does it. This really gets on my nerves as I think it’s quite rude... I could be in the shower or anything. She did it again today (granted this time we were expecting her and she did knock) so I (as politely as I could) asked her to not do that. Now my partner is annoyed and thinks I’ve been rude to her. My own mother doesn’t do that and I wouldn’t expect her to so I guess I don’t understand why my MIL does. Maybe I’m uptight lol!