It's good that you're using a good flea control product. That doesn't rule out flea allergy completely (it only takes a few bites for a reaction to occur) but makes it less likely.
The reason not all cats in a house are affected is because not all cats have the allergy.
This type of hair loss using to be called 'endocrine associated alopecia' because it was thought that the symmetrical pattern was due to hormones. Now we know that its caused by an allergy or sensitivity to something the cat comes across and is usually caused by over-grooming of the affected area. Cats can be really secretive about this so you won't always see them at it!
There is a list of diseases here:
http://www.peteducation.com/article.cfm?cls=1& cat=1338&articleid=195
Note that ones like Cushing's and hyperthroidis are not likely in a cat as young as yours.
In any event, it is likely your cat will require some form of treatment to break the cycle, so you'll need to go to a vet to get it sorted.
If there's anything on the list you don't understand, feel free to post your questions here and I'll attempt to answer them!