Cat pooping everywhere but box?

I have two cats Baby and Roxie. Roxie has only been in our home for 4 months and is using the box just fine. But baby has started to poop and pee in on the bathroom rug and under the dinning room table. I have taken her to the vet and everything checked out fine. could it be that i changed the type of litter i was using or could it be the fact we have a new cat.I was hoping to get some answers because my husband said we have to get rid of her if it countering and i really don't want to do that , I love her.