Cat (3) has rash inside mouth

Hello! My cat (3) has had a rash on the inside of his mouth for at least a week now, that seems to be getting subtly worse. Not sure if it’s bad enough to require a vet visit yet or not. Not pussy, but he doesn’t seem as playful as usual, and hasn’t been eating as fast. No new foods or toys, but new catnip crunchy treats. Any opinions? Thank you so much!

Hello!  Sometimes, inflammation of the gums can be related to dental disease and your vet should / will certainly have a look at the teeth.  However, another cause of stomatitis / gingivitis in that place can be a current, latent or past viral infection, perhaps from an agent such as calicivirus or possibly even FIV or FeLV, that flares up mildly and periodically at times when the cat is a bit under the weather.  Whatever the cause, it looks to be painful and your vet may want to prescribe an anti-inflammatory pain killer, for example, until things settle down.  Never give pain killers yourself at home because some of the very common ones that humans take without thinking, turn out to be highly toxic to cats.  A soft, easy to eat diet can be much appreciated rather than biscuits when the mouth looks sore.  I hope that your vet will be able to help and advise you further.