What causes my 9 weeks old's top teeth to stick into her bottom gum, which also looks tender?

Neonatal teeth : Neonatal teeth erupt a few weeks after birth. Normal baby teeth erupt around 6 months. The dentist should look at the teeth since they are causing trauma to the bottom gums. Sometimes neonatal teeth will be loose since the root has not developed and they must be extracted to allow the child to eat properly. Often the dentist can smooth the tips of the teeth to protect the gums.

May be teething: First, it's unusual but not unheard of for a 9 wk infant to have teeth. Second, the lower teeth more typically erupt before the top. So your baby is apparently an early teether. Her lower gum may look tender because those teeth may also be erupting soon; when she closes, her upper teeth hit the lower gum making it even more sore. It's temporary.