Be aware that there have been scams that send out fake invoices, to try to lure people into signing in to a fake website “to correct things”, thereby supplying the scammers with your credentials (phishing).
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support
Identify legitimate emails from the App Store or iTunes Store - Apple Support
First check your subscriptions page, to see if anything new was added without your consent.
View, change, or cancel your subscriptions - Apple Support
Also check the statements from your financial institution, to see if anything was deducted in line with the email.
If you see 'itunes.com/bill' or an unfamiliar charge on your statement - Apple Support
If you think your Apple ID has been compromised - Apple Support
Change your Apple ID password - Apple Support: this should keep out others from making purchases on your account, if they had the old password.