It's probably b/c you brought home the new kitties. You should do everything in your power to make that cat use the litter box, b/c it can easily become a "dirty cat" and never again use the litter box, Do the new Kitties, and The Cat share a litter box? maybe if so try to give them there own. Keep them seperated for now to allow some breathing room. I hope you find a solution to your problem.
Did all of this start when you brought the new cats home? I've always heard it recommended that you should have at least one box per cat. Some cats don't feel comfortable pooping in someone else's box, especially if they are lower on the pecking order.
Another concern is that the cat is old; stuff goes wrong in old cats (and old people and old dogs and old whatever). If it hurts to poop, some cats will associate the litter box with pain, and will poop somewhere else in hopes that it won't hurt if they try on your carpet or on your bed instead. Try to watch your cat poop, or check the poop after they do - is she straining to poop? Does the poop come out rock hard? Is she pooping every day, or less often? If any of this happens she's probably constipated, which explains the pain. I'd take her to a vet to have bloodwork done - it's a good idea to do with a senior cat once per year - as it can reveal a lot about what's going on. She might be showing symptoms of renal failure or other kidney/liver problems that can be managed if you get her checked out.
maybe she needs privacy..my cat did the same thing.
I got her a litter box that had a big top cover so its really like a big box with a space for the cat to crawl in. or just put the littler box in a closed off room , but make sure the cat knows where it is and can easily access it.
Also, don't make cats share littler boxes..eww, how would you feel stepping in someone elses poop?
or maybe shes just getting too old...my friends cat went blind and started pooping everywhere before she died.
annnd one last thing..make sure that the litter box is always pretty clean, like I said before would you like stepping in poop? gross.