Did the vet give you a course of anti-biotics when you took the cat to get checked over?

It could be all manner of things, I really believe another vets visit is in order

And please could you not pick at the scab, you could introduce infection to the sore area, and make matters worse

You could bath the area with sterile salty water in the meantime.

It must hurt when cat tries to groom her face, or rubs it against something and eventually could get infected. Vets visit as soon as possible!

Good luck.....