not a cat expert or veterinarian
but I heard that cats hesitate to use their litter box or are confused about it once moved if it doesn't have their own scent in it or urine or maybe a piece of fecal matter which labels it as theirs in their own mind similar to regular scent marking where they run their face and cheeks up against things and people
I think this is probably talked about in this video
(I find Jackson Galaxy's videos to be very interesting and many cat owners in the comments of his videos find him to be helpful)
edit: or maybe this one
I hope this helps in someway and that Sweet Pea feels comfortable soon (what a great and lovely name for a cat)
edit 2: I wonder if putting the poop from outside the box into the box and letting it sit there for a while will encourage her to poop there or indicate to her that it is her territory as well