There are two clear, major possible reasons why a domestic cat might be peeing because there is either a new baby, new dog, new cat or anything else that is new in the home and the reasons are (1) idiopathic cystitis and (2) spraying to mark territory.
Idiopathic cystitis is exacerbated by stress and it is further exacerbated by dry cat food. So if your cat is on dry cat food all the time and something big and new has happened in the home which disturbs a cat’s routine then they may develop idiopathic (unknown cause) cystitis which is indicated by small drops of urine, sometimes bloody, around the home and sometimes in the bath tub and so on. The disease happens in people as well. You will have to rule out an infection and if there is an infection then you will have to decide what antibiotic to give. A veterinarian might do ultrasounds and x-rays to determine the possible cause. What might help is transfering your cat to a high quality wet food diet and make sure they drink as much water as possible (boiled fish plus added water?).
The other cause is spraying to mark territory and the underlying reason is exactly the same for idiopathic cystitis namely, stress and upset. If there is constant upset like this and a domestic cat becomes stressed because of it, indeed chronically stressed because of the ongoing presence of a new cat or other animal in the home, then to reassure himself a domestic cat might spray inside the home to tell others that he is occupying his territory (home range). He may renew the urine as it fades in strength and so the spraying will be continuous. It is not normally described as urination because is not done to eliminate urine but to mark territory; to leave a calling card to say this is mine.
Territorial marking is a territorial cry for help. You have to get your cat to feel more relaxed, if you want a quick fix you simply remove the cat or dog (you can’t remove the baby) and your cat will revert to the state he was formerly in if this is an instance of cat spraying. You can try a pheromone spray which is meant to calm cats but I’m not that confident about them. Jackson Galaxy would say that you should make sure that the home is fully ‘catified’ by which he means it’s a place where your cat can express natural behaviours which will certainly help but at the end of the day your cats got to get on with the new animal or the new dog.
They will adapt naturally over time so if you can bear with it and provide your cat with some space where they can feel secure then gradually your cat will adapt to the new dog, cat or baby and the problem will probably go away.
What to do?
So what do you do? You have to go back to square one really and introduce your cat to these new ‘creatures’ who have invaded his territory and disturbed his life. That’s a whole new minefield of problems I would say because it’s quite hard to do it in the right way but if you are turning the clock back after your cat has become stressed it might be more difficult. There’s lots of pages on the internet and on this website about how to introduce a new cat or new dog or new baby to the resident cat so the information won’t be hard to find.
Without being in any way critical, the problem points to a failure in domestic cat caregiving. Cats are extremely sensitive to routine. They like stable environments which are calm and predictable. If a person is going to introduce something new they have to incorporate into the process their domestic and the feelings of the domestic cat. They are sentient beings and they are definitely attached to their home, more so than people. People tend to think that cats and dogs can be brought into a home where there is a resident cat without a problem (it may be okay) but home owners would not let a human stanger come into their home to stay and accept it. There would be hell to pay. They’d be massive problems.
The reasons I’ve given on this page are based upon the fact that this is happening to a healthy cat who is peeing normally before a change takes place with the introduction of these animals or child. Of course a cat can pee inappropriately for all kinds of other reasons such as injuries and illnesses other than cystitis but that discussion is not really suitable for this page.