There are two reasons a cat pees outside of the box: medical or stress. 85% of the time, it is a medical problem. They have very few ways to let us know there's a problem, and that is one of them! The best thing to do is to get kitty to a vet ASAP.
I don't know if you've read the forum rules yet, but rule number two states
Originally Posted by TCS Forum Rules
2. Please make sure to spay and neuter your cat. Unless you are a professional breeder and your cat is part of a professional breeding program, please educate yourself to the importance of spaying and neutering by the time your cat is 4-6 months old. By spaying and neutering you enhance your cat's quality of life and improve his or her health. You are also proving your love for cats because in acting as a responsible pet owner you are minimizing the problem of cat overpopulation.
Many people have a moral problem spaying a pregnant cat. That does effectively mean aborting the kittens. However, you do need to know that is an option. Whether or not you chose to do this is up to you. But you chose to let your kitty have the kittens, you may want to start looking for homes for them now.
They should be allowed to stay with her for 10 - 12 weeks (12 is actually best) before being adopted out.
If you need to search for free or low-cost spay/neuter services - to see if there are any in your area -(the kittens can safely be spayed or neutered at 7 weeks old, so it really is best to spay or neuter them before they are adopted out), or if you need to look for a rescue to help you, or if you want to look for someone who may be able to foster them, you can search at these sites:http://www.pets911.com
At PetFinder, you can scroll through the cats up for adoption - each cat has an organization listed next to it. Each organization has a link to a page that will have contact information: http://www.petfinder.com
You can also search for low-cost spay/neuter services here: http://www.lovethatcat.com/spayneuter.html
Once a cat has peed outside of the box, the place kitty has peed MUST be cleaned with an enzyme
cleaner. They are typically available at most pet stores. This is the ONLY type of cleaner that will clean up the smell to her nose, which is what matters.
The area she peed must be COMPLETELY soaked. On a mattress, you have to soak it really well. Wherever you apply the enzyme cleaner, leave it for 10 - 15 minutes, blot up the excess, then leave it to air dry. When on a carpet, we cover the area with aluminum foil until it is dry (this also helps discourage kitty from going there again). On the bed, we cover it with a towel and make the bed.
Any soiled clothes or bedding must be washed with the enzyme cleaner added to the laundry like you would fabric softener.