can I get a new concussion from full braking with a car?
 

Today I was driving behind a Truck, so I could not see what happens in front of him. Suddenly he stopped and I hit my breaks really hard to come to a full stop from about 100 km/h
My head felt strange after that and I got dizzy right away.

Could that have damaged my brain again? I finally thought it started healing, because I had some good days in the last weeks.

No, that was not a concussion. You likely caused a spike in your blood pressure to your head causing your symptoms.