Is it a normal type poop (vs. a soft, diarrhea type)?
You could attempt to put a litter box directly on that spot. See if she uses it or uses another part of the floor. This will give you a clue as to the next step. If she uses it. keep it there and gradually move it to where you eventually want it to be. (slowly is the key here). If she doesn't use it then she is rebelling against something else. That is where you must think of ANY possible change, even if it seems remote.
type of litter
scent of litter (companies change their products so you may think that you are buying the same thing)
Times of in room/out of room
feeding time
cleaner (for boxes, or carpet etc)
the list could go on endlessly
And of course you may want to have her examined by a vet./
Best of luck.