Kitten Won't Poop in Litter Box?

We just found a stray kitten that had been living behind a building. She pees in the litter box, but she poops everywhere else. We keep her in our room and in the bathroom at night. Her litter was a cheap kind that had big pieces of litter in it. We switched to one that is more like dirt, like she would of been using outside, but she still won't use it to poop in.

Also she has a very swollen stomach so we took her to the vet a couple days ago. They gave her pills for worms, but we aren't exactly sure yet if that's what the swelling is from, we have to wait to see if the pills do anything. But the doctor said that shouldn't cause her to not use the box to poop. Can you please help us?

By Kayla Allen