A transaction on my card was made but not by me what can I do?

DO NOT click on any links if you received an email 'notifying' you of this charge. Report the email or other notification to Apple as explained here: Phishing & Other Suspicious Emails

If you got this notice in your bank account billing statement, see this article: If you see 'itunes.com/bill' or an unfamiliar charge on your bank, credit card, or debit statement - Apple Support

If it did get billed to you, then secure your Apple ID to make sure that nobody else was using it: If you think your Apple ID has been compromised - Apple Support

I would also see this article: About Apple Pay - Apple Support

How can I check my recent transactions?

When you view your card in Wallet, you’ll see the last transaction for each credit, debit, or prepaid card, including Suica in Japan and Apple Pay Cash in the United States. To see more, tap the card and tap . Then tap the Transactions tab. Depending on your bank or card issuer, you might see only transactions made from your device. Or you might see all transactions made from your credit, debit, or prepaid card account, including all Apple Pay devices and your actual card. You can also see Suica transactions in the Apple Watch app on your iPhone.

Some banks or card issuers provide only initial authorization amounts to Wallet, which might be different from the final transaction amount. You might see different transaction amounts in Wallet and your statement from locations such as restaurants, gas service stations, hotels, and car rental companies. Always refer to the statement from your bank or issuer for final transaction details. For Apple Pay Cash, you can get a statement sent to your Apple ID email address.

Suspend the card in question and contact your card provider to deal with the charge. Sorry you had to go through this.... :/