lack of exercise end up resulting this problem..
take her out before her meals for atleast 40mins, running is very important for your gsd + walking helps dogs to make a mental bonding with owner.
Feed her twice a day 11am and 7pm
1hr running,1hr walk everyday.. best for your dog
there can be many reasons behind this problem consult dr for bloodtest.
and u can try pedigree professional (black packing) good for your pocket
eukanuba,royal canine (expensive)
Dog Behaviour Specialist