Do you suspect theft or fraud on your account? Call your credit card issuer.
If it's a case of unauthorized usage, by law you're only liable for the first $50 in unauthorized credit card charges. Most card companies cap your liability at zero dollars if you report the charges soon after discovering them.
If you suspect someone has hijacked your card or card number, mention that in the call, cancel your old card and arrange for a replacement.
The issuer may even be able to keep a closer eye on your account for a period of time. You could also go through current charges to make certain all of them are yours.
In cases of unauthorized use -- where you suspect theft or fraud -- it can be helpful to file a police report (if possible) or fraud affidavit with the Federal Trade Commission, says Chi Chi Wu, staff attorney for the National Consumer Law Center.