What should you do now?

You selected both correct answers. On multi-lane roads like the Autobahn in Germany, you must always keep as far to the right as possible when you are not overtaking (this is called the "Rechtsfahrgebot" or drive-on-the-right rule). After overtaking a slower vehicle, you should return to the right lane as soon as possible when it is safe. In addition, if a situation requires you to reduce speed (for example, if something unexpected happens or as a precaution when traffic conditions change), braking may be necessary. You should not move to the left lane unless you need to overtake another vehicle, as unnecessary driving in the left lane is against the rules. So, by continuing to drive as far to the right as possible and braking if needed, you are following the correct German traffic regulations for this situation.