At 17 weeks your baby still has quite a bit of cushioning from the amniotic fluid, so a fall on your knees where the impact didn't directly hit your bump is generally lower risk than a direct abdominal fall. That said, it is really hard to know for sure without a proper check, so the safest move is to call your gynae or head to KKH just to get assessed and put your mind at ease. Better to go and be told everything is fine than to sit at home second-guessing yourself.