Why does my kitten keep peeing on my bed?

I have 2 stray kittens, a boy and a girl, that I rescued about 2 months ago. They both use the litter box regularly; however, the girl occasionally pees on my comforter, roughly every other week. This week, she peed once on the comforter, and once on the other kitten who was lying on the comforter. I can't figure out why and/or how to get her to stop. I keep the litter box very clean, and it is in the same spot it has always been in. I've tried negative reinforcement as well as rewarding her when she does use the litter box. I've never had cats before, so I hope this is not regular behavior for cats. Can someone please offer some insight because I have run out of ideas? Thanks!