Potty trained kitten peeing on beds?

Hey everyone,
We got two kittens (sisters) who were both potty trained when we got them, now both 5months old. They were really great with there litter box. A couple months ago one of the kittens started peeing on our beds, not always just some times, we were concerned and rung a vet, took her down and she gave us antibiotics in case of an infection. Its seemed to work.... at first, she has started again =(
There are no other animals living her except the two kitten sisters (who get along very well) and a fairly stress free environment.
This is a problem for only one of the sisters, we arnt sure what to do now!
Thanks you!

I had a cat who started using my bed as a potty. I knew the reason though she accidentally was locked in my room when I went on a trip. After several failed ideas we hit on one that worked. We moved her litter box back in my room and kept her locked in the room with me at night. The only comfy place to sleep was with me at night in the bed so she stopped peeing on the bed.