Check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.
Apple ID device list -Check to see where you‘re signed in.
Apple ID Password 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.
What happens when you try to change your Apple ID?
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 hasn’t published any guidance for using an address again. Normally, if your e-mail address was associated with an ID in any way, for example as a Reachable at or Rescue e-mail address, then it can't be reused with another Apple ID for approximately one year. I have also seen posts that stated Apple told them there was a 30 day waiting period before the address could be used again. You will need to get another e-mail address to use as your Apple ID or you can try waiting 30 days to see if that works.