If someone gets a mild concussion, are they able to go back to normal after 2 days if symtoms clear? how small of another hit could make it worse, could a very small bump?

Symptoms: Return to play is based on return to play guidelines. Typically if asymptomatic and if light aerobic activity does not illicit symptoms return to play is OK. https://www.cdc.gov/headsup/basics/return_to_sports.html. A bump on the head in itself does not effect this decision. Sometimes swelling can be associated with a depressed skull fracture. That would make a difference.