I have 5-6 year old german shepard/chow mix.
She's a steady groomer for certain regions; however, I believe that the butt has been constantly getting more attention for longer periods of time. To the point if she's walking down the street she will take breaks to lick there.
Which I thought as many others do, is a normal habit for dogs.
She also has allergies flare up every now and then and will start to itch randomly, but I have been given antihistamines from the vet for them. 
However these days the underside of her belly while she is standing will constantly get scratched using her hind legs followed by licking. even after an antihistamine.
one person said that she might have something inflamed inside of her (requiring an operation) that causes this extreme itching and licking in these areas. any thought?
(Also she loves rolling in grass and swimming - but she is taking K9 Advantix repels and kills everything,, and of course I give her her worm medication)

I've never seen a dog break off from walking to lick it's bottom - I would think there's something making it very uncomfortable. The first thing would be to check her anal glands aren't impacted.

As for the general itching - what are you feeding her?

I've been feeding her the same brand that she's had since a puppy: "Nutro" line, Natural food.
I have recentlychanged to the older adult version that has added glucosomine for joint mobility as her hips had started to bother her. also she gets half a tablet daiy of glucosomine & condroitin (for dogs); although it is liver flavored, she is a very picky eater and needs it to be stuffed inside cream cheese/cheese  or liverwurst. however these treats she has always been given, so it shouldn't be a "new" thing she's reacting too.
Maybe she is sensitive to eating cheese, bacon, liver type foods now? Is people food so unhealthy for dogs? It's not given on a regular basis or anything, although she does get spoiled by "grandma" when visiting

I'd be tempted to have her properly tested for allergy. Your vet should be able to organise this.
:)

"Maybe she is sensitive to eating cheese, bacon, liver type foods now? Is people food so unhealthy for dogs?"

I would stay off the human food also - I know you say that she has been having these treats for a while now with no problems but to be honest I'm having second thoughts about feeding some 'human' food to my OH and children - the cr*p that goes in it is beyond me.  People and dogs can develop 'sudden' reactions to food that they have eaten for a while, wheat / gluton and lactose intolerrances are on the increase and one of the most common to suddenly develop and allergy/intolerrance to.

Tara x

I'd also be concerned about the possibility of Anal furnuculosis in a dog with GSD in its breeding.....

Good point.

Best to get her tested for Diabetes as this can be an early sign.Also change her meat to fresh meat (cooked) or raw tripe.I always feel  so sorry for itching animals as it is so aggravating for them.