It is probably a urinary tract infection. She started associating her pain with peeing to the box where she wants to go. Then she finds that it hurts no matter where she pees. Then she tries to tell you. Was she still pooping in the box?

OK, sounds very anthropomorphic, but probably pretty true if this is the problem.

We had an old cat do similar. Turned out the cat had developed diabetes and her kidneys were in trouble.

No matter what else. Sounds like a vet bill is headed your way....

They get rid of excess blood sugar by peeing it out--and they become not very particular about where they do it. She's been on insulin for almost a year now, but we've mostly solved the peeing issue with extra litter boxes in strategic places.

It may be something treatable.

Likely a uti so, when you gotta go, you gotta go. Check with vet as she is likely in pain too.

Don’t want it to turn into a blockage…

It would be a good idea to check with the vet in case she has a UTI, but old cats can get confused and sometimes that includes litter box problems. https://pets.webmd.com/cats/guide/old-cat-behavior

if she couldn’t make it to the litter box.

Her last week I kept her in the main bathroom with the litter box in there and she still couldn’t manage it. I knew she was ready to pass.

Hopefully yours is a different issue. I wish you the best.

She needs to get to vet.

Our old calico, Zoey, died of severe dementia when she was 20. It came on slowly, first she stopped grooming herself, then she got a lot more withdrawn and unresponsive, then lost control of her bowels and stopped eating for the most part. We;d have to take her to her food and show it to her and that worked for awhile, then she stopped eating altogether. She wasn't in any pain and vet said we could let her pass peacefully at home, so that's what we did.