I'm going to ring in here. I agree with you, at your age, you deserve some privacy, consideration and respect. Your mom should be giving that to you by knocking on the door before entering and waiting for permission to enter.
Having said that, other responders have also made several valid points. I agree that this is a) not a big deal, b) this is her house not yours so you have no real rights, c) she is unlikely to change at this point so you just need to live with it.
What I would try is to sit down and talk with her, maybe with a third party she respects, like a grandmother, aunt, etc. Explain to her that you feel you are entitled to some privacy and ask that she knock and wait for permission to enter. If she won't concede then you have to live with this until you can move out.
One thing you might consider is getting a screen for your room. Do your changing behind that screen.