You can give Benadryl to a cat. The dose is about 1 mg per pound of body weight but I have never needed to use that much. It can cause drowsiness. If your cat weighs 6 pounds then I would start with not more than 3 mg. I would give even less. It always better to start very low and add a little more later. At a normal dose it ca be repeated in 8 hours. You might also want to try DERMA-IONX to decrease the congestion and stimulate the immune system. I would also give probiotics to fight any infection and stimulate the immune system.
Please let us know what your veterinarian finds.