You should always consult a doctor before giving a cat over-the-counter antihistamines. Some antihistamines may not be suitable for animals, and could hurt it greatly.
You should talk to your vet before giving your cat any over-the-counter medication, because many human medications are deadly for cats. However, some antihistamines are safe for cats, so your vet may have a recommendation.
Why would you think you could properly diagnose what was wrong and administer a human-reactive drug in the proper dose to your feline? Are you crazy? Never give you cat people medicine. A trained Vet is the proper treatment for your cat.
