I put flea and tick med on my cat. Ever since I put it on her fur where I placed it has been twitching. Is that normal? I went to the pet store to get frontline plus for my cat and my puppy. They were out of frontline plus for cats so i bought Bio Spot (generic brand). Which they said it was similar but I guess not as strong. Should I be concerned?

I applied ‘biospot’ medication for flea and tick control to the back of the necks of my two cats, both over 5 lbs. The medication has an oily nature and ever since it was applied the cats have been scratching as far as they can reach with their hind limbs.
It seems they are not tolerating it, the fur on the back of their neck is falling out from all the scratching. But otherwise they seem normal, that is why I haven’t washed the area. Has this happened to your cat? Do you suggest I wash it off? (it’s been 24 hours since application).
Thanks for reading!