You should contact your credit union right away. It is important to notify the credit union promptly upon discovering unauthorized charges in order to limit your liability. 
You may contact the credit union by phone, but the credit union may require written notice about unauthorized charges.  The credit union should have provided you with information on how to report unauthorized use of your credit card.  The address to write to may be different from the address for sending your payment.

After receiving your notice, the credit union must conduct a reasonable investigation of the claim if it may hold you responsible for any part of the unauthorized use.

Actions that a credit union may take in reviewing a claim include:

• looking at the transaction in light of other purchases, 
• reviewing if the goods were delivered to the residence or place of business, 
• comparing signatures, 
• requesting a police report, 
• requesting documentation to assist in validating the claim, 
• requesting a signed written statement from the cardholder or authorized user, and 
• requesting information about the cardholder's knowledge of the person who allegedly used the card or of that person's authority to do so.

The credit union must notify you of the results of its investigation.