How much licking is "excessive", possibly indicating a skin irritant?

I read somewhere that if a kitty licks excessively, it could point to a skin problem for them. How do you know if they are licking too much or their skin is irritated? I have a furry, what I think is a Chinchilla Persian cat. She can barely get to her undercoats. She doesn't usually appreciate brushing but I am trying to get her used to is so her coat is not matted. She's letting me do a little longer, little by little, without running away. But is it normal for her to groom herself just after me grooming her with a wire brush or a petting session?  

Thanks in advance!

In my experience, Ritz grooms herself after I pet her. I don't know exactly why, except maybe to get my scent off of her. (I don't take it personally..
I think there are other signs if your cat is grooming herself excessive, like loss of fur.