True allergic reaction is possible but unlikely. Your dog may be sensitive to the product so even the correct amount (topical flea and tick products are usually sized by weight) may cause an overdose situation. Neurological signs-altered mentation, tremoring, or lack of coordination can be seen with toxicity in several of the flea products. Make sure the product you are using the product correctly and for the correct weight range. Smaller dogs can sometimes be more sensitive to the chemicals in flea products. Next time this happens wash the product off your dog and see if the behavior stops.
Also some dogs do not like the feel of the product on their skin—so they are not reacting to the product just the sensation of oiled fur it creates.
Lastly if your dog is able to lick the flea preventative he may be experiencing a taste reaction. Again not an overdose or poisoning he may just not like the taste of it—as though he ate spicy, spicy hot sauce.
Ann Falk, DVM