Getting a sick not in Netherlands

Where you on a holiday?
The way your employer would see it is that the injury was not work related, did not happen in Germany so not sick days.

I went to Netherlands after my 5 days work. Because i work 12 hrs a day as outpatient nurse. I had back pain already those days due to lifting of the patient but i thought it would go after 5 days, which by that time i would have to go back to work again, but it got worse so instead of travelling back to germany on Sunday, i reported sick the friday because i still feel pain and i couldnt risk lifting and it might get worse. So Wednesday i went to see a dr in Amsterdam and sent them(my employer) the note. But still they didnt seem to recognize it coz in the Dienstplan it was still written as off days instead of sick days. so now i have -55 hours of work for this month.

I'm certainly no expert in German labor law but for an employer to switch scheduled work days missed because of health reasons to being vacation time sounds illegal. That it happened in the Netherlands or that the validation was from a Dutch doctor would seem totally irrelevant. I would confront the employer and if necessary contact a union representative if you have one or possibly a lawyer.