If she is acting "normal" for her and did not lose consciousness, observation would be fine. You can give her tylenol if she complains of pain (either by telling you or by holding her head, etc.). Ice over the bruise will limit the swelling and bruising. Remember, bruises tend to be worse the next day than they are the day of the injury.
If her behavior changes or she gets very drowsy (more than usual when she is sleepy), you should have her checked out. If not, by bedtime tonight she should be fine.
I hope this helps and thank you.
I would wait until bedtime. Does she usually take a nap? When?
Catnaps are OK during the day. Just watch her and wake her every few hours or so.