Someone changed my xbox gamertag without me knowing

log in to your Microsoft account locate all your purchases. then once you find it you should see a refund request. fill out all the information and you will receive an email saying that you have been refunded or reason's why you were not refunded.

It must have been something naughty lol. What was it?

Could it have been considered offensive or in breach of codes of conduct and changed by enforcement?

Check HERE
for info.(link to enforcement site)

If however, you feel that your account has been accessed without your knowledge, check out the steps HERE(solutions for hacked
account)

In the meantime, since you dont think it's an enforcement issue, I would take steps.to ensure.your account isn't compromised. At the very least, change your password

My xbox gamertag had changed without my authentication and now I have to pay to change it back.

I would like to revert all the unauthorized changes to my account.

If the change is not on your billing tab chances are enforcement changed it for you because of reports