How do I handle my MIL being mean?

If there isn’t a UTI present, then please take breaks from your MIL. It’s important for your own mental health to distance yourself once in a while.

You handle it the way you would want her to treat you, if you where in her shoes.

And learn everything you can about dementia so you understand them better .

Teepa Snow on YouTube

What do you mean how do you handle it. You already SAID you know she has dementia and doesn't mean it. Isn't that good enough?
You say that until today she has been good for SEVEN YEARS????
Hon, that woman must be a SAINT.
So treasure her, forgive her, love her, and understand her.
That's it. There's not another thing to do with dementia. Unless you want to shrug and give a laugh or a smile. My best out to you. Carry on!
And by the way, if she continues to get meaner this week and has been so loving heretofore, those advising it is time for a test on that urine are stone cold correct!

I agree you should take her to Urgent Care for UTI check. If she doesn't want to go you tell her a therapeutic fib that you think she'll accept (like she's going for her "annual wellness check", etc). UTIs are treatable with antibiotics. If she comes home with meds you must be the one dispensing them to her until she is clear of the infection. UTIs are very common in seniors, especially women, as their pH changes, their physiology changes, etc. not necessarily hygiene related.

If she's really resistant to going, I would call 911 and tell them she is very confused and you think she has a UTI but won't go. They will get her to go.

Have her tested for a UTI immediately. They can cause behavioral changes in elders with no other symptoms.