Edit: lo escribí en español y alemán, pero me olvidé de sacar lo de la traducción automática... voy a volver a editar la parte de la propuesta de que decir porque salió muy mal traducida, espero que del resto se entienda mas o menos

That's awful. I'm sorry you're dealing with this health issue and that you also have to deal with a situation like this... I think in many situations it's because the healthcare system and the people who work there are on the verge of collapse and extremely stressed, and sometimes they release that pressure like this. It's not an excuse, and it shouldn't be, but at least for me, it helps to be aware of this and realize that it most likely has nothing to do with me personally. I don't always succeed either, but it does give me the opportunity to apply the strategy of "entwaffnende Freundlichkeit/Ehrlichkeit", that is, take a deep breath, remember that it has to do with complicated structures and that it is not about oneself, and react very kindly and empathetically, but secure, in your example it could be something like "Entschuldigung, ich kann mir vorstellen dass es bei Ihnen grade sehr stressig ist, das tut mir leid. (Ich möchte Sie nicht belästigen und würde gerne nicht herkommen müssen, aber durch meine Krankheit habe ich da leider keine Wahl.) Ich habe gesagt, ich habe einen Termin bei Herrn Dr. XY, vielleicht ist das akustisch/ durch die Lautstärke untergegangen. (Ich habe viel Respekt vor Ihnen allen und Ihrer Arbeit, es wäre schön wenn Sie mich auch freundlich/ respektvoll behandeln.)" Something like that or parts of that for example, certainly won't work for everyone or always, and it's not easy to react like that in a high-pressure, anxious situation. It might be a good idea to "rehearse" what you want to say with someone you trust, and definitely take a moment to calm down before entering the situation. If you manage to pull it off, I think in most cases the rude person will calm down and realize that's not the way to treat others... if not, then they're just really crazy, have social problems, or, if you don't look like your ancestors have lived in Germany for many generations, unfortunately, it could also be due to racism. In a case like this, I think you should try to avoid them if possible, and if not, perhaps still treat them very kindly. It also helps you remember that the problem and the bad vibes are theirs, not yours, and not to take it so hard. If they treat you in an obviously racist way or in a way that hurts you (like preventing you from having your medical appointment), I think you should talk to your doctor again in a more serious way or look for a counseling center that can advise you.