Do take her to the vet for a full MOT.
It may well be that she is simply being fussy, or she might have some level of a food intolerance. It could be that she needs a different food, that the one you are currently feeding doesn't suit her - either for taste, protein source, undigestable ingredients, consistency, or (hopefully not) medical reasons. Because of the latter it is always best to first see the vet as it is easier to eliminate medical reasons off the bat and so better focus your efforts.
After 20+ years of feeding the various members of the Muttley Crew I thought I was well-schooled in canine nutrition. Enter Mr Persnickety Puddle - it took me working with the vet for about 6 months to determine if there was a behavior problem, a problem with the food, or if he was seriously ill.
(In the end, we learned that he had IBD - and that started the long search for a suitable food.)
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So - before making any suggestions, a bit of info is needed:
What are you currently feeding your dog? Listing the ingredients and analysis might help.
How much are you feeding, and how often?
What was she eating in the Tierheim? Did they report any problems?
Second - what are her elimination habits? How often, how much, formed or loose, what color? (Your vet will need to know this.)
What are her drinking habits?
Any vomiting? And if so, what - digested food, mostly undigested food, bile liquid? And when does vomiting occur in relation to meals and exercise?
Any signs of coprophagia (poo eating) hers or that of another dog or cat?
Have you looked thoroughly at her mouth? Any signs of abrasions, swelling, tooth problems, etc?
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I know how worrying it is when a dog stops eating. Paws crossed for your little 'un.