My toddler bumped his head pretty hard. should i worry?

Monitor closely: If there was no loss of consciousness, no change in how the child is acting, no vomiting, then it is probably OK. Watch for any change in mental status (confusion, walking or talking differently, just not acting right) as well as persistent vomiting, or excessive sleepiness.

LOC or Vomiting: For a ground level fall, if he did not lose consciousness or vomit for 2 hours after the impact he is probably fine. If he did, get to and er.