I was walking with a heavy backpack and fell forward. is it easy to get a concussion if i had one before. head leaned and hit top part of backpack.

Head injury?: Without signs of head injury I would not be concerned. CBI is a additive, progressive disorder from multiple injuries and not caused by a single incident. The events have not been clearly outlined (to my knowledge) but common sense would suggest that repeated injuries would be associated with at least some minimal central signs like visual or auditory disturbances like seeing stars, ringing, etc.

BRAIN GAMES: Concussion means 'to shake violently' and 'shaking' the brain can have additive effects. So yes you can re-injure your brain.