chewing on own self

Question
I have noticed my lab chewing on her own hind leg area!!  I can't figure out why she is doing this.  She is whistling as if it is an irritation she is trying to get rid of but there doesn't seem to be anything outwardly showing that would be an irritant (fleas, ticks, rash, cuts, scrapes...nothing)  I have seen dogs scratch themselves with their front teeth but she is putting her hind leg entirely in her mouth.  Is this possibly an alergic reaction to something or what?  I haven't changed her food, routine, drinking water or exercise amount.  Is this possibly a sign of her going into heat soon?  There is no sign of swelling on her genital area?  I am concerned but clueless on this.  

Answer
Almost anything can be ''normal'' when a female is coming in heat.  Nobody in their right mind keeps an intact female unless they are planning on breeding.  Breeding is very difficult to do responsibly.  Go to www.akc.org and read about responsible breeding.  Go to www.petfinder.org and see how many dogs in your area need homes.  Get her spayed before she does come in season.  

Diagnosing the cause of the chewing and stopping it is difficult.  Dogs can eventually develop an allergy to the same food they have been eating.  A vet visit might or might not turn up anything.  It could also be boredom.  You need to enrich her life.  If you are leaving her at home all day, consider doggy day care.  Otherwise you can look into obedience competition, agility, flyball, free form dancing, etc.