this can have several reasons, the most likely being an allergy. As Kai mentioned this could be against flea salvia but a food allergy is just as likely.
(By the way- if you're not sure if your cat has fleas place the cat on a sheet of white paper and give it a thorough tickle
. Then take a damp cloth and rub across the paper- if you see red dots thats the blood in the flea droppings).
You should treat it against fleas if it has them and in paralel start looking into the possibility of an allergy. Unfortunately this can take some time and detective skills. To do this you should start writing a diary for your cat where you note down what you've been feeding and everything else you've noticed. For example- if it is a pollen allergy the exessive grooming will be dependent on the season. Another common trigger is house hold dust.
A food allergy is also right on top of this list- in this case your cat may be allergic to a certain type of meat- say chicken or rabbit or another common ingredient in cat food such as wheat.
Your best bet there would be an elimination diet
containing only one uncommon meat source such as horse (ostrich, kangaroo, venison), which should be fed strickly for a minimum of 6-8 weeks. Another possibility would be hypoallergic cat food, where the meat source is changed in a manner that the cat's body doesn't recognize it as an allergy source. If the symptoms become better you know you're definately dealing with a food allergy- the next step would be finding out against what exactly. This is done by one by one testing a new meat source for 6 weeks each to find out which are OK and which you must avoid.
(It could also be that your cat is feeling pain but can't get to the source of it- and to distract itself from this pain it self grooms. As an allergy is so much more likely I'd look into that first)