Some cats may be allergic to either the bite or to the venom of an insect. Allergic reactions can range from minor swelling at the injury site, to hives and anaphylaxis, writes Hanie Elfenbein, DVM. She suggests protecting cats from bugs that bite.
Source: Pet MD. Link. A minor allergic reaction can often be effectively treated with a cold compress and diphenhydramine (Benadryl) at 1mg per pound of cat. Topical treatment is not recommended because the cat may ingest it through self-grooming. If the swelling persists, the cat appears painful or is not behaving normally, or if there are any concerns, it is always best to see a veterinarian.