What am I supposed to do after finding out that a few bills I have are counterfeit?

I would contact your local police department. The United States Secret Service investigates counterfeit money. Your local police department should hopefully get in touch with them regarding the counterfeit bills. Hopefully you can also help them out by telling them where you got them. If they are not going to investigate the matter, maybe they can help you out in getting money back from wherever you got them from.