It's really hard to avoid doing this.

Non-autistic take, and Ive been on both sides.
If you really aren’t comfortable, bring it up with the person later. I have done this, and had this done to me as well.
“I was thinking about what happened the other day, I said I was Ok at the time but I am really not Ok …
<explain>
”
- That is fantastic advice for people who are capable of direct responses like that.

People-pleasing and excessive conflict avoidance. Been there before.
- I live there.
- Is that an autistic thing ?
  
  When you spend your formative years trying to 'fit in' (like most autistic people) you quickly learn that standing up for yourself is a failure in your attempts to not stand out. Those chidhood habits are incredibly hard to unlearn.
  This meme isn't an exclusive trait for autistic people, just one that many autistic people struggle with.
  
  Definitely more common with autistic people. There's a theory of autism, Intense World Theory, that posits the fundamental concepts of autism make the world is too intense for us. We perceive things more intensely, we process things more intensely, we remember things more intensely, and we focus more intensely. Since the world is tailored for NTs, we are more likely to develop a traumatized personality which results in fawning/people-pleasing. Orion Kelly has a video I like on fawning, though he doesn't discuss the Intense World Theory.
  As far as my own personal beliefs at this time, I think that autistic people are constantly told from childhood that their perception of the world is incorrect. If we don't agree to succumb, we are punished. This makes us dependent on others to tell us how the world works. As such, we don't develop confidence in social situations and become submissive to others.

I hate the compulsion to be nice to people that I'm not comfortable with.

Part of the social contract is forgiving people so we can all move forward. I don't think you can go back and say something was not ok retrospectively after having said it was ok, without breaking that social contract and going back on your word....but next time you interact with the person you can ask them not to do it again.
There isn't much point in dwelling in the past and relitigating it, just work on making the future better without causing trouble.
Ex: Branda, it's totally cool last time, we all understand, no worries about it. But please don't get drunk at the next party on Saturday, we have to set a good example.
- This is frustrating to read because it kinda sounds like, "Just don't be bothered by it, act like everyone expects you to and say the right thing. It's not hard." One reason people on the spectrum dwell on past interactions is precisely because they're trying to work on making the future better without causing trouble -- that's the hard part when social norms or interactions are difficult to wrap your head around.
  
  The trouble with relitigating the past after you said everything is ok, is now people don't know if your ok when you say your ok. You become a "problem" and can get avoided.
  By all means, say something isnt ok when it isn't. But if you say something is cool, and we are ok... Then own it. Not because your emotions aren't valid, but because everything is better when people can trust what you say.
- I think it depends on the thing that was done, whether it harmed someone, and if it harmed you. You wouldn't want to say, "it's cool you hit me and broke my nose" even if you forgive them, and especially if the reason you brushed it off before was out of fear of escalation. In cases of harm, they've already broken the social contract.

Some worries man, some worries.
- Getting rid of britta.
  https://youtu.be/bv5XlcdAyoM?si=tMAjKlGFEjKmNTdK
  
  Wait Britta's in this?

i got a friend who says to me now, "I get where you're coming from and that's really kind of you, but it isn't really all good, is it?" and it's been e n l i g h t e n i n g

Once I figure out who's shitty (especially after delayed processing days, weeks or months later) I just stop positive engagement with them. They never have to know what I figured out. Leaves room for other to fill the void and life is better.
If I really don't like them I tell them exactly why. I've got sputtering responses that fill me with warmth years later each time I remember or tell the story to someone I care about.
People are not often bluntly confronted on their own shitty behaviour and there is a power over them when they know your know.
- That is quite similar to what I do. I just stop seeking them out.

I had a situation like this at work even though I am not autistic. My boss verbally assaulted one of my colleagues in a very inappropriate way but I did not say anything. Neither did my colleague. At the moment I felt uncomfortable and also expected my colleague to react.
I actually thought about this a lot and even called my union to ask for advice. Should I wait for it to happen again or could i go and bring this up several weeks later?
It is never too late to bring it up and say: 'you know that thing you did yesterday/last week/last month? I didn't feel all right with that even though I didn't say so'
I also added that although it was not targeted at me, that I would really get offended if I was treated like that.
In my case it improved my relationship to that boss and we can have more open discussions than my colleagues have

Is someone the barber? 😁

I do this even when it's something extremely minor. "Yeah they brushed me as they went past and said sorry but what if it was more then a brush and a stumbled and fell on the road? Some people..."

I relate to this a lot.

How often are you interacting with people that do something horrible enough to warrant this being on your mind??
- Oh. I see you don’t work with people. 😆

I always thought that was just a me problem...
it does have it's upsides though. Revenge is so much easier when people have their guard down.
On a more serious note; yeah I revoke "no worries" occasionally. try not to do it too lightly, but if I feel it's relevant... I'd rather cause some drama than have all this unspoken bitterness buildup