And cue the insults and insensitive comments. Seriously, what is wrong with people on this forum! If all you can say is "I hope my son doesn't marry someone like you" why don't you just shut up? But you love getting your thrill from anonymously insulting posters online. Gross. This forum could be a safe space but people who behave like this won't allow it. Thank goodness there are some very sensible forum users who provide great advice and take the time to explain their reasoning.
OP, I don't think you're being unreasonable. She should have asked first. You have every right to want to be the one in charge of changing nappies, feeding, and everything else concerning your baby. Please disregard the nasty comments.
You're probably feeling ashamed of those protective feelings but at the same time angry that she took the liberty to change the nappy without asking first. The good news is that you don't even need to explain WHY you feel the way you do, you're entitled to being protective of your baby. This has nothing to do with the quality of the baby-grandmother relationship like others have suggested.
Someone said she probably didn't ask because she sensed that you would turn down her offer, so she just dived right into it. If that's the case, that's some overbearing behaviour right there. Perhaps you need to think about healthy boundaries.
That said, I would not sweat it or bring it up with her just now, she won't understand. In the future, I would simply speak up and say "thank so much, but I'll do it" with a smile on your face. There, drama averted. If that creates drama (which is unlikely), then it's time for a conversation about boundaries. And please also remember that grandparents sometimes are SO eager to be helpful that it can be annoying for new parents, even if it comes from a good place.
Also, I just want to gently remind you that it's a really tiny tiny thing in the big scheme of things, so it's probably best to forget it and move on. 💐