You didn't mention how old she is or how long you've had her so I am assuming that she is a young kitten and fairly new to your household. Kittens are like children - sometimes they get distracted with play and exploring and have accidents. If you haven't already done so, you need to quarantine her in a single room where she is within easy reach (hopefully within sight) of the litterbox, food, and water, until she learns where these items are. In addition, as has already been mentioned, cats like doing their business on soft, absorbent materials - the laundry basket is a very popular alternative to the litterbox. On top of that, down is an invitation to pee, so if you have a down comforter it may be hard to stop her.
First thing I would recommend is that when she first wakes up in the morning promptly put her in the litterbox, and keep her there (or in the room, e.g., a bathroom where the litterbox is) until she goes. Same with going to bed at night. Reward her each time she goes (regardless of the time of day) so that she associates it with a pleasant experience.
A couple of good enzyme cleaners are Anti-Icky-Poo and Unique Distributors - both can be purchased online. You will definitely need to clean (soak) the mattress with the enzyme cleaner as well as the linens. Avoid using any ammonia or bleach based products as these will just encourage her to go in those areas that have been cleaned by these products.
I agree with the above that this is probably simply growing pains that she is going through, and a learning process that she will eventually conquer. Please be patient with her, I think you'll find as she matures so will her bladder, and her litterbox habits.