My german shephard eats james well beloved food but she has lost 3kg in 8 days I have a scheduled blood tests with the vet but I am really worried, she is not showing any signs of illness, also she is wormed with drontal and fleaed with advocate

Her weight could be stabilizing because she is getting more mature, but if you have a scheduled appt. with your vet, then don't worry.

Tick

She could be getting more exercise without you really knowing. My lab could eat 8 cups of food and eat thousands of treats and not gain a pound, but she would lose weight very fast because she was so active just playing in the yard. Don't worry too much if you have a vet appointment booked, it's probably nothing serious!

Any changes in diet or eating habits?

Usually around 12-18 months a lot of dogs (especially males) will go on hunger strikes and just barely eat, dropping a lot of weight.
I went through that with my poodle when he was 12 months and my Chinese Crested pup is a week shy of 12 months and he has started not eating now.

Around that age is when they also start losing their 'puppy fat' and gaining muscle, it might be time to up the exercise and undertake some more rigorous activities. Exercise also stimulates the appetite.