Why is my mother rearranging things in the fridge and kitchen?

Stack your things on a separate shelf maybe in see through containers designed for kitchen use in the fridge. Designate your shelf space with these containers that you can purchase online. Tell mom these are yours and do not touch them. As for kitchen shelves find another space to store your things or get a lock on a cabinet to keep curious hands out of your stuff.

Perhaps you can negotiate a shelf or two for yourself.

or she may just be looking to see what you have …

At that age, they tend to be obsessively self centered. The only thing that matters is Her Stuff, not yours, so she'll put it wherever she damn well pleases. Hmpff. Ornery and in charge of the entire household, that's mom. Hire some in home help on her dime and get out of the house for the day to refresh yourself. Repeat as needed. You matter too.

If that's the only complaint at 98 I think you are doing marvelously well, frustrating as it might be. This sounds a bit like a little OCD (obsessive-compulsive) activity that can sometimes surface when a person feels "control slipping" and is attempting to try to take control. Memory may be less acute and she may be attempting to get her "stuff" arranged so it is more easily handled.
Provide some tasks. Folding laundry, etc. that she can do sitting down and enjoying TV or music.
And good luck!