I'm sorry she has been doing this and know you will be concerned, and likely frustrated.
It is not that uncommon for an older dog, especially a large breed, to start to toilet more inside. There can be a few causes including;
- Joint disease making it harder for them to make it outside and to posture
- Canine dementia, which can affect many aspects of life and also causes a toileting regression
- A urinary tract infection
- A neurological disease
At this point, a vet check is sensible in case she may require some medication in the form of joint supportive anti inflammatories (to improve mobility and encourage movement).
The vet can check her all over, performing a neurological and orthopaedic assessment.
They can also discuss if perhaps canine dementia is a factor. There is no test for this, but they should be able to recognise the signs (if present) from what you tell them. While there is no treatment, affected dogs can benefit from supplements and a medicine called Vivitonin. It is also important to keep their routine predictable.
For now, offer as many toileting opportunities as you can.
Be sure not to tell her off for going inside, as this could make things worse. She won't be doing it out of 'protest' but more because she cannot help it.
Do clean up any stains with an enzymatic cleaner to prevent the odour lingering and the urge to 're soil'.