My 10 year old cat just started using the bathroom in the house. Ideas?

She may have a urinary infection.

Has she been to the vet? An older cat often does this when she is experiencing a health concern, like diabetes or a UTI. Cats also may start to do this if something is upsetting them like a dominant cat outside or a change in the household routine. But my guess is she needs a check up. Mine had diabetes and was always very clean until she started to become ill. At ten, some conditions start to rear their ugly heads. Apart from that, you might try offering a litter box if she has been going outside. Perhaps something scared her when she was going potty and she feels vulnerable out there. There are cat litters that have an attractant in them which encourages the cat to go there. But first, I think a trip to the vet is in order. Good luck.

She may have a urinary track infection. At 10 years she needs to be checked out by a vet. I wouldn't discipline her until I knew she wasn't ill. She may be trying to tell you something.

many elderly cats have F.U.S. ( feline urinary syndrome) where crystals precipitate ( drop out) of the urine into the bladder. it irritates whenthey pee and they start taking quick little pees and avoiding the litter box because they associate it with pain. get your vet to flush her bladder and it could salve your problem.