Frontline and hair loss

I have been using Frontline on my cat and noticed yesterday that the hair has become very thin in the area when I apply the Frontline.  Has anyone else seen this and what do you recommend?

Will be taking her to the vet next week anyway, so will ask the Vet too.

Read the label on the box to see if it has a warning that can happen. I guess side effects and allergies to that are possible.

that happened to one of mine, he got a bald patch after I used Frontline. Funny thing is that I've used it since and it never happened again.