When a cat starts eliminating inappropriately, it's either due to a medical reason like a urinary tract infection, crystals (tiny particles which develop in urine that is not acidic enough), bladder stones, or the cause is behavioral.
First, you have to rule out the medical possibility, so your cat needs to see the vet as soon as possible for a definite diagnosis and treatment, if necessary. Antibiotics will be prescribed for a urinary infection and dietary changes and/or nutritional supplements to acidify the urine will be suggested, if crystals are found. http://www.sniksnak.com/cathealth/fusfaqs.html
If a cat experiences pain upon urination or defecation, she will come to associate the PLACE of eliminating--the litterbox itself--with that pain, and then choose to eliminate in other places. Cats in this situation usually choose 'soft' materials to go on, thinking this will avoid the pain. Carpet, throw rugs, bathmats, clothing, sofas and beds are usual favorite places. If she's using the litterbox sometimes, and urinating on the rugs sometimes, this may have become a habit, if she is not troubled by any underlying medical problems.
In order to prevent your cat from returning to the places she's already soiled (if she smells it, she'll keep thinking they are appropriate toileting places), you can use an enzymatic cleaner to completely remove all traces of stain and odor. Nature's Miracle and Zero Odor are good products for this purpose, and can be found in Petsmart/Petco, most pet supply stores, and online. For more info and to view all related products, please see: http://www.naturemakesitwork.com/home/index.phphttp://www.allivet.com/Equalizer-Carpet-Stain-and-Odor-p/26942.htm www.zeroodorstore.com http://www.redhotcarpetcleaning.com/en-us/simple-solution-faq.html
You did a good thing in buying new bathroom rugs. For the moment, try not to keep any rug down in the bathroom, until you know the source of her problem and it's resolved. When you or family members put down a bathmat for a bath/shower, don't allow her in the room. If her litterbox is in there, move it right outside the bathroom for that short time.
Cats can also sometimes start eliminating inappropriately if there has been any kind of change in their lives or environment, as this causes stress and anxiety. If the reason for eliminating outside the litterbox is NOT found to be medical, it would have to be treated as behavioral.
Cats are creatures of habit and do not take kindly to change. If you have had any new people added to your household, or if a 'usual' person has left, if you got new furniture, carpet, appliances, another animal, etc., or if your usual daily hours at home and/or away from home have changed or you've gone on vacation recently, any of these situations could cause your kitty stress.
Feliway/Comfort Zone for Cats, is a cat-appeasing pheromone which can help to calm stressed cats and curb urine marking and/or inappropriate elimination. It comes in both a spray and plug-in diffuser. The diffuser plugs into an electrical outlet like an air freshener, but has no scent to humans. For more information, please see: http://www.catfaeries.com/feliway.html
Major pet supply stores like Petco/Petsmart carry Feliway, you can also order it online, and some vet offices carry it.
There's another product which I think may help you get your kitty back to using the litterbox all the time--it's a clumping kitty litter called 'Cat Attract'--it contains herbal attractants which encourage use of the litterbox. It too, may be found in major pet supply stores.
You did a good thing in adding a litterbox because some cats prefer to urinate in one box and defecate in another. The rule of thumb is usually one box per cat, plus one. If you have a multi-story house, a box on each level is also appropriate and usually prevents accidents. Of course, keep all boxes immaculately clean at all times, as some cats are very fussy about this, too.
Keep in mind that first it would be a good idea to bring your cat to the vet to determine if the cause of eliminating outside the litterbox has a medical basis.
I hope all goes well, and this problem is resolved for you and your cat, very soon!