I think your cat has ear mites. Watch it carefully. You might find something black in there. There are so many ways to get rid of it. Put Vicks in its ear or use another way that is very effective. Take one-fourth cup warm water and add half teaspoon of boric acid in it. Add two teaspoons of tea tree oil. Further add one-fourth cup white vinegar and only two drops of dish detergent. Soak cotton ball into this solution and then squeeze it to remove excessive solution. Now press it into its ear and take it out. It will help to remove mites from there.

 It sounds like mites to me also. When a cat has ear mites, the ears almost always smell bad. If it looks crusty and dirty, like black or brown, on the inside of the ear you probably need to treat it before it gets too bad. You can buy treatment from a petstore, or your vet. They're not too expensive, and your can will feel much better.