same cat, same place, same litter, but something has obviously changed. the
quick and cheep suggestion is add another litter box and see if that helps.
If the cat is showing ANY other symptoms, or if the second litter box
doesn't help, get thy cat to a vet.. could be the start of a UTI.

Abrupt changes in behavior are often a sign of physical problems in
cats. They aren't big complainers. I'd take her to the vet to rule out
any problems before looking at this from a behavioral POV. Urinary tract
infections are, from what I understand, the #1 cause of problems of this
nature. They are also easily treatable.
She's only 6, but I believe certain joint ailments can affect them even
at that young age. If she has movement problems, she may need a larger
box, or a more shallow one. There's lots of things to look at, so get
her to the vet to see if it's a physical problem before we look at other
things to do. If the vet gives her a clean bill of health, then start
looking at what else might have changed.
Oh, if she's declawed, there is a slight chance (very slight, depending
on the skill of the vet who did it and the method used) that the nails
are growing back embedded, which would cause pain but be hard for you to
notice. Have the vet check her feet for any bone/joint problems in her
toes if she's declawed, just in case. It is very rare, but can happen.

HTH

Could she perhaps be not lowering her rear end enough that she misses
the box when she tries to pee at the edge or corner?  I have that
problem with one cat, and have the litterbox inside a slightly larger
storage bin to catch any misses.