I am sorry to hear you are worried about your cat. My name is Rebecca. I am a veterinarian and will do my best to help.
This sounds like some kind of allergic reaction. If she goes onto a screened porch, it could be environmental (pollen, etc.), parasites, or a food allergy. Just because you don't see fleas does not mean it isn't fleas; when cats develop flea allergies it can take only a few fleas to set them violently scratching, and often they swallow the flea when they lick and groom themselves. I once saw a bald cat that was itchy and I mean bald; almost no hair at all. Never saw a flea on that cat but when the owners applied some Advantage once a month, all his hair grew back. It was flea allergies all along.
Have you changed her food recently? Food allergies can make cats itch and scratch.
Other things that can make a cat itch intensely are autoimmune skin conditions, microscopic parasites called mites, and internal diseases such as liver disease.
She is telling you something is wrong, and she sounds like she must feel bad. I would have her checked out by your veterinarian if this continues. It won't do any harm to put some Advantage or Frontline on her, and it might help.
Let me know what else I can answer or help with.