No, she is not a toddler in dog terms -- she is an adult, the equivalent of a person around 20 at her age (the 7 year/dog year thing isn't really that accurate). Dogs have housetraining problems for a reason. Either they weren't fully trained to start with (which isn't your case), or there is a physical problem -- eg medical -- or a behavioural issue causing it. By behavioural I do NOT mean she is *in any way misbehaving* -- it means something has changed in her environment, meaning her food, her home, time she gets with family, new arrivals (dog or human), changed furnishings, could be anything -- that makes her anxious and prompts this behaviour -- I would guess a strong possibility is as she has matured into an adult and also goes outside, she just doesn't care to go in a small litterbox (see below), which is probably also too small, if you use a cat tray (cats are half the size of most cavaliers). For dogs, defecation marks a place as their territory and is comforting -- it isn't something they do to mess up their environment, for example.
But the starting point should always be -- get her to a vet. You need to remove all possible medical causes -- some of which can be urgent and serious. Always, always, when behaviour changes or unusual symptoms show, at the very least call your vet for advice on whether or not to bring the dog in.
Vets will give appropriate advice.
Then once you
are assured it isn't medical, examine what has possibly changed in your house. If her toiletting has changed you may need to do remedial work -- eg NO punishment, which can actually increase her tendency to hold i and go inside. Use rewards and go back to taking her out, supervised, at regular intervals and do not leave her out of your sight where she can go unseen.
I have always heard very mixed results with litter training dogs and in general this approach works better with very small toy breeds, not a dog the size of a cavalier -- and I would bet this is your (NOT Maya's!) problem. It probably works better with puppies but wouldn't really be a normal behaviour for dogs and you'd need to provide a very LARGE tray -- probably something more like a small kid's sandbox in size. She may have just ceased to associate the litter box as where she has to go. That means you will need to housetrain all over, with a larger box -- or move her towards going outside (an adult dog can hold herself during the day for several hours -- if no one is home with her for so many hours, day and night, that you must use a litter box, I would seriously question whether this is the best living situation for a dog -- any dog -- and if she is indeed left alone for huge periods, that is probably causing the problem as well. This is a very social breed amongst an animal that is already very social, and being alone all day with no companionship, no human interaction, would be pretty mentally devastating for a cavalier, so maybe adjusting her life and yours so they intersect more would be the best approach, or get her to doggie dayvcare during the day so she has something going in in her life. If someone is there during the day/evening, and she just sometimes goes inside, and sometimes outside (which is probably the case?), she may just no longer separate going in the litter box from going outside the house (eg outside the litter box) as being any different from in the house. Maybe the box is too small and makes her uncomfortable. Most litter boxes designed for cats -- which are a lot smaller than cavaliers generally -- would really be very small spaces for dogs to manouevre and she may just be tired of using one? My guess is that it is actually having to use a litter box -- and perhaps whether someone is there -- that is the issue.
PS I do not have garden and have to walk my dogs daily for their toiletting. On average they get 3-4 walks daily for this. It isn't a major issue to get them out at such intervals. For two of those trips out I tend to only go up to the corner for them to wee, not a full walk. It is quite easy to manage toiletting outside even in an indoor living situation with no garden.