Thank you for the further history about wee Star.
I am glad that we can rule out any injuries that might arise outside (since there are just so many possibilities) and that we can put the potential for bacterial induced abscesses caused by another cat at the bottom of our list of concerns for her. That said, we do need to be very careful when we see such severe lameness associated with swelling in a kitten Star’s age.
In these cases, we can see these types of signs associated with muscle bruising/strain but in young kittens we can also see more serious fractures even with short falls. And in Star's case, if she may have had a fall or even a tumble down stairs or off furniture then fractures will be a concern here.
Since it is Friday evening, this has just occurred, she isn't showing severe pain, and you have not noted any signs of an open fracture ( where we see open wounds with bone exposed-- as this would be the one type of fracture that would require immediate addressing), you can choose to restrict her activity (no jumping/climbing and strict rest) and monitor her overnight. If the swelling and her lameness is still profound in the morning then we'd want to follow up with her vet to have them examine her. If they are suspicious of fracture, then an xray should be considered to let you see what has happened.But if they can rule out fracture and confirm that this is soft tissue based, then often strict rest (in a room with no surfaces to jump on), warm compresses, and feline safe anti-inflammatory medication (ie metacam) can often settle the signs of muscle strain and sprain.
As a precaution, I would also suggest not feeding her any more tonight or until after she is seen (or we are sure that she doesn't need to be seen). She can continue to have access to water but skipping further feedings will make sure she has an emtpy stomach if she does need to be admitted for xrays tomorrow morning (and that a full stomach doesn't delay getting this sorted for her).
Overall, we do have to consider that this could be a fracture. If you think her pain is severe or there is an open wound with bone exposed, then we'd want her seen tonight by an ER vet. But if she is only showing mild discomfort, then you can choose to rest her tonight to limit further damage and aggravation of the injury. As well, if Star is amenable to it, warm compresses can be used as well.But if do this overnight and she continues to be lame, then we do want to have her vet have a feel, rule out any fractures, and to get her on some kitten safe pain relief to take away the soreness and discomfort she is telling us about with her limp.
And just to note if you do want to have her seen tonight, I wanted to mention that most veterinary practices here do have contingency plans for emergency care for their patients. This means that if you ring the practice, they will likely have a message to direct you on how to contact their emergency service. And if you don't have a vet you can find one local to you, you can check RCVS register (LINK). or you can check here to find your local Vets Now (LINK) who are open all nights/weekends.
I hope this information is helpful.
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