bald spots

i recently put a flea collar on every one of my cats....i have 4.one of them started digging at it to get it off,and as a result,has a large bald spot on her neck.i noticed when i took the flea collar off that she was also balding on the back of her neck,where the collar sat.im guessing that maybe she was allergic to the powder?well,now ses got a blad spot about the size of a half dollar on her back,right where her tail starts....is there anything i can or should to to either stop this from happening now or prevent it from happening later?the vet says she isnt allergic to anything that he tested her for...could it just be the fleas,like how dogs get "hot spots"...could hers be a hot spot,too?