I have a severe allergic reaction to my cat's flea infestation. Is there anything that would prevent my swelling.

How often did you take Benadryl? It should be repeated every 6 hours for optimal effectiveness. But if Benadryl or cortisone cream did not work, there may be no over-the-counter medication that does. It is wiser to consult a doctor. You can also come directly to the pharmacy for a consultation. They will be able to better assess the severity of the lesions and the need for medical consultation

Be aware that Benadryl causes drowsiness. Do not mix with alcohol or do activities that require alertness.

You could then try cetirizine (Reactine). It is an antihistamine that lasts and provides 24-hour relief while rarely causing drowsiness

It feels good. Also apply cold compresses, bath with oats or bicarbonate. And above all try to avoid scratching. The lesions should disappear quickly if the cleaning has been done. If it persists for a few weeks, it might be useful to treat with an insecticide as for body lice but for the moment this does not seem justified and will not treat your itching. It would risk irritating your skin even more.

I found this interesting resource from the Faculty of Veterinary Medicine of the University of Montreal.