Apple ID being used

What makes you think your Apple ID is being used?

Regardless of the reason, change the password of your Apple ID. The next time the "other person" tries to use your Apple ID he will be prompted to enter your password, which he will not know. Furthermore, should this "other person" learn your new password, you will receive an alert on your phone telling you of the attempt. Simply tap on "Don't Allow" to prevent the other person from getting into your account.

You should Check your Apple ID device list (https://support.apple.com/en-us/HT205064) and remove any device that shouldn't be signed in using the steps in the link.

If there are none that shouldn’t be signed in, then you should not have a problem at this point. But, if an unauthorized device is signed in, Change your Apple ID password https://support.apple.com/en-us/HT201355 

Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

And see these security-related Apple Documents to deal with cases of unauthorized sign ins and other security matters:

  If you think your Apple ID has been compromised https://support.apple.com/en-us/HT201303

Security and your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

Using app-specific passwords https://support.apple.com/en-us/HT204397

 You can also add 2 Factor Authentication to your account for more security. If the attempts continue, you might want to consider a name change.

Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

Apple ID -Two-factor authentication          

 Two Factor Authentification availability.     

 Apple ID - Using app-specific passwords.     

Apple ID - Two Step Verifications FAQ