My male GSD (7) has A.F and its just flaired up 'again' and we are tweaking his meds to get it under control.  Its very sore and he is definately grumpy and uncomfortable.  Today, he was stroked by a child who he knows very well and he lifted his lips and growled at him.  He is an extremely well behaved dog who is fantastic with other dogs and lives with my two children and sits outside school with me and has been patted and spoken to by 100's if children since he was a puppy.  He is known as the gentle giant. His meetings with children are always supervised and has been nothing more than a total gentleman.  We have a new puppy (6 weeks ago) and he has accepted her but doesnt over love her like my other GSD bitch (she has totally mothered her since day 1) We all get on just fine. She was with us when it happened - could it be him protecting her?  It has upset me that he has acted in this way as its so out of character.  Whilst cleaning his bum yesterday he was growling alot at me and was very tense - I understand its very painful and I obviously cant explain to him that I am trying to help him. 

Could this change in his behaviour be due him being so uncomfortable? I will not be taking him to school anymore as I just can't risk him upseting another child which could lead to a complaint - I am sure he would never bite, I was sure he would never growl so I am confused as what to do for the best. 

> Could this change in his behaviour be due him being so uncomfortable?

Yes, definately.
I agree it is not a good idea taking him to school, it will protect children and also ensure your dog doesn't have to feel so threatened.

Don't think of him as a different dog atall, he is a dog that is feeling vulnerable at the moment. As you have seen, feeling vulnerable can make a dog defensive over things that wouldn't usually bother him (hence why I don't think anyone should ever trust a dog 100% - you never know how they are feeling).

Just be aware that he is feeling vulnerable and be very carefull about who you allow to stoke him. Personally I wouldn't let anybody stroke him or get too close to him. My last dog used to suffer from bad ear infections and I have no doubt atall that he would have bitten a stranger if he even thought they were going to touch him any where near his ear, so to play it safe, I wouldn't let anybody touch him while he had a bad ear, not even my own children. (He was generally a very friendly, loving dog, but pain from a bad ear really made him defensive). It's not worth the risk and it's not fair on the dog to be put under that undue stress.