I would look for two things when a dog is chewing his backside. An Anal gland problem or a flea allergy. The anal glands are located on either side of the anus and if they become full or impacted it will cause discomfort for the dog. A flea allergy will cause constant chewing until the area is raw and devoid of hair. A dog does not have to have many fleas to cause a reaction. It only takes a few to hop on , bite and hop off as the dog is actually allergic to the flea's salvia. Here is more on both of these.
anal sacs http://www.marvistavet.com/html/body_anal_sacs.html
flea allergy http://www.petplace.com/dogs/flea-allergy-dermatitis-in-dogs/page1.aspx
I never saw a flea on my dog either, but he chewed his tail raw from a flea allergy. If it is a gland problem there is not much you can do until the vet expresses the gland, unless you want to try expressing it at home, the site I sent you above will show you how .
You can stop the dog from chewing by using an Elizabethan collar so she can not reach the area. If the area where the hair loss is red then you can apply a triple antibiotic ointment to aid with healing. Also Labs are prone to hot spots, are you familiar with those? If it is a hot spot then washing frequently with antibacterial soap and applying the antibiotic ointment will help, but you still need to keep the dog from accessing the area.