I suspect your flea treatment was not effective in maintaining flea control.
When cats itch it is caused by fleas 50% of the time, food 40% of the time, and all other allergies and problems ( inhalant allergy, environmental allergens, house dust mites, bacterial infection, etc) 10% of the time
Fleas and food must be eliminated before thinking of tests for everything else.
Purchase a flea comb and comb ALL pets daily for 5 days looking for live fleas or the black flecks of flea dirt that looks like ground pepper. You cannot rule out fleas in cats by just looking under the hair. You must comb them. If you see any evidence of fleas on anybody....treat everyone.
In the case of flea allergy, it is not the individual flea bite that they are scratching. When a flea allergic cat is bitten by a flea, they have a reaction that causes them to react everywhere.
It is not 100% but flea allergy tends to be a back end problem, while food allergy tends to be a front end problem.
Any of the prescription once-a -month products available from your veterinarian are effective. Remember though that all monthly flea products work great for 3 weeks, well for four weeks, they do not work for 5 weeks. Treat all cats once every three weeks for at least 2 treatments. Do not waste your money on over the counter products, they do not work. Comb regularly after that and treat as indicated.
To stop this current reaction, you can use Claritin or Zyrtec allergy medication at ¼-1/2 of a 10 mg tablet once or twice daily. Start low and increase dose if needed. This is not a substitute for eliminating what is causing the reaction if possible. Crush into powder and mix in 1tsp favorite wet food. I will tell you that if it is food allergy, no medication and no amount will stop the reaction. The only way to stop food allergy is to eliminate the offending food.