what could be the cause of my cat’s ear infection?

I took her to the vet because she was scratching her ears and they started bleeding. The vet said the inside of her ear was great and there were no ear mites. They put her on cefadroxil (oral suspension). Her ears got a lot better during the two weeks she was on it. I stopped the dosage on Wednesday (that’s when the vet said to). However, it’s getting worse (slowly) again. I have noticed some white powdery stuff (on the outside of the ears) and the outside of the ear is balding again.

So, I am worried what is causing the problem. What can I do?

the vet is closed tomorrow and Sunday! So, suggestions on what I can do till I talk to them are welcome!

thanks in advance