Hi, my dog Lady has allergies.
First, about allergies. Dogs develop allergies the same way humans do, by exposure. It's usually the 2nd or 3rd exposure to an allergen which will trigger the reaction. If you've never before used OTC flea drops, then they likely are not the cause of the itching.
To treat Lady's itch, we use Detol applied topically. Dilute 1 tsp (5 ml) Detol with 100 ml warm water and put into a spray bottle (works best on areas with little or no fur, like the belly). If you don't have a spray bottle handy, you can also saturate a cotton ball and apply it that way (works best on areas with dense fur). It's a bit smelly but seems to sooth the skin.
There are many reasons why your dog could be itchy so keep at it with the vet until you get an answer. It took us over a year to get to the bottom of Lady's allergies. Also, be aware that skin conditions and allergies are very common among Cockers. As with humans, allergy testing in dogs is not necessarily conclusive.
Hope this helps.