You might want to bag up a fresh stool specimen and take it with you on your next vet's visit and have it tested. I paid a lot more for my pup, who came from a breeder who gave me records of multiple rounds of worming. When I had my vet test her feces, the results indicated she had roundworms, tapeworms and giardia.
I understand that puppies are susceptible to parasites. Just because they were treated for something a month or two ago doesn't mean they haven't reacquired a parasite or perhaps a prior treatment wasn't successful.
My pup was born on 7-15-18. She was presented as being from an ex-large line of GSD's, so she may be growing more rapidly than some puppies. She weighed in at 40 lbs., approx. 1 week ago.
I know there are a lot of different opinions on type of food. I've been feeding my pup Royal Canin's German Shepherd Puppy Chow. I mix in a protein, such as some freshly cooked poultry or meat. My pup usually gets around a heaping cup - 3 x a day. For some meals, if she seems extra hungry, I give her an extra serving. I, also, give her a daily joint supplement.
PS: Did you meet both the parents? Are they both papered GSD's?