As a cat owner, it is sometimes very hard to know when you should see your vet if your cat has been acting strangely lately.  However, if you notice certain types of cat behavior you'll need to see your vet right away!

If your cat exhibits runny eyes, is lethargic(low on energy) exhibits a loss of appetite or is not drinking you should visit the vet immediately. 

Also, you should consult with vet if your cat exhibits any sudden change in behavior including struggles to use the litterbox, constant diarrhea, constant vomiting, sudden change in litterbox habits, bleeding or struggles to breathe.