Hi Sandy,
My mother did the same thing, both with food and with her Glaucoma eye drops right before I moved her into the assisted living facility (where I arranged that the nurse would do the eye drops). In this same 1 month time period, Mom suddenly didn't know night from day, the time on the clock meant nothing, and she forgot the different uses of the pronoun "you". I'd ask "Are you hungry?" and Mom would look so confused, point to me, and say, "You???"
I'm not saying that your MIL will do the same, but be on the lookout for other indications of memory holes. I removed all house keys and car keys from sight after Mom tried to leave the house at 11pm (shower cap and cold cream in hand along with her coat and purse!

). The next day I installed keyed deadbolts and hid those keys so Mom couldn't get out of the house at night since the clock and black night sky were no longer meaningful clues to her. I also locked the basement door so she couldn't go there without my knowledge because I was concerned that she'd fall down the steps. You may have to install child-resistant cabinet door locks in your kitchen and lock up food and medicines, too.
It's scary to try to anticipate what you need to keep them safe, but watch your MIL's behavior for what to do next, then head her off at the pass!
If your MIL's name isn't on the list at an assisted living facility yet, it's time to do that much. When she cannot be left for a minute or leave you alone for a minute and you cannot sleep because she's up all night, you may need that option.
((((hugs)))). I know what a heartbreak this is. Don't wear yourself to a frazzle until you get an ulcer, heart palpatations, sky-high blood pressure or something like that before you realize, as many here have, that you cannot do it all, no matter how great your love or how big your heart. No guilt! You're important, too.
More (((((((((hugs)))))))))), Barbara