Couple of questions.
Does your cat wear a regular collar?
Has your cat warn a flea collar recently?
Does your cat have fleas, check for flea "dirt" or their eggs, not just adults.
All of these are possible causes, and there is more which could be affecting it. Is there any hair loss where this rash is? Is the rash really red, or does it have open skin with scabs on them?
I think it would be a good idea to take your cat to the vet, specially if this has been going on for a while.