5 month old kitten has sudden change in behavior and potty habits

We got our 5 month old Maine Coon mix a month ago. He has used the litter box until today. I also have noticed he has become less social. He use to be all over me when I got home kneading and purring but now I can hardly get him to sit with me.

Today I caught him peeing in our living room. So I cleaned it up then cleaned the litter box and put fresh litter in it. While I was at work my husband called me and said that Scooter has pooped in the living room. I need advice we just had him at the vet two weeks ago and he has a clean bill of health!

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My Thoughts:

I know the vet certified him OK two weeks ago, but something has changed. Sudden behavior changes (less social, withdrawn, lethargic, and so on) plus inappropriate urination and defecation (total litter box aversion), are signs that he's possibly ill or has been exposed to something toxic. The only thing I would recommend is what I would do if this happened to my cat... call your veterinarian's office and tell them what's happening.

Please let us know how he progresses.

-Kurt