I have a 12 weeks old german sheppard and she's growing fast but is very skinny. I feed her accordingly to the instructions on the dog food bag (very good quality) and 3 times a day. When it's time to eat it looks like she hasen't eaten in a month.
Any comments?

When was she last seen by the vet for vaccinations? Has she been wormed?

3 weeks ago for shots and yes she was wormed

Shepherds are notorious for being skinny until they reach full maturity.

It's always best to keep a young puppy on the lean side rather than overweight, it can cause stress to their joints and bones if they are carrying too much weight.

Do you have any photos you could post?
I find that 90% of the time what an owner thinks is underweight is actually perfect for the dog.