Hi there Doug!
I'm a veterinarian in Ontario, Canada and I would like to help you and your cat.
What you are describing sounds like feline psychogenic alopecia (FPA). With this condition, the cat "barbers" the fur (chews it, bites it or licks it) in one area to the point of baldness (alopecia), or even until there is bleeding. It is often due to an underlying stress.
With what you are describing for your kitty, I totally agree that it may have started with a few fleas, and then kept going even when the fleas were gone. FPA is a bit like nail-chewing in humans - it is done because it makes the cat feel calmer, as a way of dealing with stress. As with people who chew their nails, it may continue even when it must be causing pain if the skin has become raw.
Some breeds seem to be a bit more prone to FPA (Siamese, Burmese, Himalayan, and Abyssinian) but it can occur in cats of any age, sex or breed.
Before your cat can be diagnosed as definitely having FPA, your veterinarian would need to rule out a few other possible causes of hair loss in the belly area. These would be fleas, ringworm, Demodex (mange), allergies and bladder inflammation (some cats lick over the bladder area when it is uncomfortable).
There is no specific test for FPA. Instead, it is a "diagnosis of exclusion" which means that when we have ruled out physical problems, then what is left is a stress response problem. I am highly suspicious, however, of FPA because it occurred with the stress of being spayed.
I would wonder if there is anything in the last couple of months that may have been stressful for your cat? A move? A new room-mate or another pet? Construction or renovations? From a cat's perspective, even rearranging the furniture can be stressful! Is she stressed about not being allowed outside?
Unfortunately, there is no quick cure for FPA and it is something that one has to "manage" as opposed to "cure." Treatment involves trying to minimize the stress in your cat's life. One of the things that I would strongly recommend is a Feliway Diffuser system. This is a plug-in device that sprays a cat pheromone into the air, helping to calm the cat without drugs. Here's a link to more information:
http://www.catfaeries.com/feliway.html I find the plug-in diffuser to be the most helpful since it works even when you are not there, which is when your girl is most likely to feel stressed and lick her tummy.
Also, keep in mind that this may flare up at times of stress for your cat - such as moving, a new room-mate, new pets in the home, and so on. Using the Feliway diffuser at those times may really help Kitty to adjust.
If this does not work, your veterinarian could prescribe a number of different "Prozac"-like drugs for cats. As with humans, sometimes it takes a while to find the best medication for relieving your cat's stress, and you might have to try 2 or 3 different drugs before finding what works for your cat. Usually, medications are only used when the problem is so severe that the cat is damaging her skin.
I will include some links to further information about FPA:
So, in summary, it sounds as though your cat has feline pschogenic alopecia, though physical problems would have to be ruled out first to be able to diagnose this. Treatment aims at minimizing stress, using a Feliway diffuser and possibly using medication if needed for worsening problems or known upcoming stress, such as a move.
If this has been helpful, please hit the green "Accept" button. I will still be here to provide more information if you need it!