Why “find my iPhone” went off?

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.

Link to contact Apple Support in the article.

Apple ID has been compromised.      

Security and your Apple ID.     

Checklist: If you want to see if anyone else has access to your device or accounts 

What do you mean "went Off".

If your iPhone modified/jailbroken?

If not then one cannot remotely hacked your iPhone.

Changing your password should have logged them out. The sync to show you that they were logged out may have been delayed if they weren't using their device when you changed your password.