A delay between the actual date of purchase and the time you receive the notification may happen depending on the platform. 
If you have received a charge for an item you don't remember purchasing, could you please review your "Purchase History" via the links below to see if the billing date matches any of your recent activities?

AppStore：https://support.apple.com/en-us/118212?device-type=mac

Google Play：https://support.google.com/googleplay/answer/2850369?hl=en

If you are unable to identify the purchase date, we kindly ask that you contact your platform provider (Apple or Google) directly for further assistance.

In addition, it is common for family members to make purchases using the same device. 
We kindly ask that you check with your family to see if anyone might have made the purchase.