My mom's cat, George suffered something similar for years after getting antibiotics from her vet. He had red spots, used to scratch and bite himself so badly he had wounds, and had seizures.
The vet had him on monthly steroid shots - I think Prednisone, and he never got better. Another vet agreed with the treatment, so my mother thought there was nothing else to do, and refused to stop the treatments.
After years of listening to the vets, and no improvement after about $5000 of treatment, we talked her into seeing a holistic practitioner because poor George was miserable. She advised no more shots, no more antibiotics, and no more stressful trips out of the house unless it was an emergency.
George cleared up completely in about a month.
I don't know for sure this is the case with your cat, but if this started after the antibiotics, I'd be suspicious.