Talk to your manager about it now.

What Brenda said. Get out ahead of it.

Yep.

agrees with brenda 100%.

brenda, btw, you can quote my last post in your performance evaluation for this year.

It's a thing that technically I shouldn't have done, but normally wouldn't have been a big issue (SOMEONE is supposed to do it, but not me--in many cases, the person would've been "oh, yeah, let me do that again in an official way", or possibly "Hey, you shouldn't do that, please don't again, let me do it officially"), but in this case the people are awful to deal with, and in bitching about other stuff I was asking them to do, decided to fight back by being like "OMG she did this!". So, yeah.

(And I'm not really sure what to say to my manager about it, other than "yeah, thinking about it I shouldn't have done it, and it was something of a brainfart for me to do it, rather than just tell them to do it, but they were being useless and not doing anything I told them so I wanted to see if it existed before I added it to the long list of things for them to do")

I think a slightly revised version is what you say to your boss. something like "I wasn't sure if they did it so I did. I realize that was a mistake." Definitely bring it up before they do.

I would say "just wanted to give you a heads-up. I took care of X because I wasn't sure it had been done. That's not the protocol, and while normally it wouldn't be a big deal, in this case the staff a X are upset about it and will probably bring it to your attention."

That way she's not's blindsided by a big complaint that she doesn't know how to respond to. And plus you look all responsible and accountable and shit.

in addition to what lisah said, I wonder if there is a way to be proactive later: like since these people are being nightmarish, is there a way your boss can offer suggestions or help in dealing with them?

In my experience, immediate managers are pleased to have someone say Mea culpa on their own recognizance. Most managers don't enjoy the kind of meeting you're dreading any more than you do. If nothing else, you become known as someone who owns their own oops.