doesn't want to go for a walk outside anymore

Hi, I'm new here. I'm feeling very worried and I'm hoping someone can shed some
light on this. Over the past month or so my kitty hasn't wanted to go outside
for her usual walk around the yard. Even with my coaxing her with treats, she
turns right around to go back into the house.
Her background: she's 20 years old, she gets 125ml of fluids every day, and a
little more than a month ago she had an in-growing claw taken care of at the
vet.
There's no change in any of her kidney symptoms-eats ok, no change in pee
frequency etc.
She seems so reluctant to walk down the front steps and go for her usual
snooping around the yard.
She's had some stiffness in her joints for years now but she still goes up and
down the 14 stairs inside the house-as recently as today. But why no walking
outside?
I'm puzzled and worried. I'd appreciate any help or suggestions.
Thank you so much,
Susan & "Meetoo"