my gamertag and was changed for no good reason

You would need to try submitting a case review at
http://enforcement.xbox.com The enforcement team are the only ones in charge of forced gamertag changes.

Hi mate, I would check the email that is associated with your gamertag and see if you were issued a forced name change

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You could also check to see if a purchase for the change was made on your account here:
commerce.microsoft.com

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and if you were forced to change it you might be able to find out more about it here:

forums.xbox.com/.../41.aspx

I really didn't do nothing and my gamertag is not harmful nor inappropriate

There's just 3 options on a gamertag change, the free first change, paid change, enforcement change, any other change means you are using a different email that the one used the first time

Since you are using the correct email address and you also mentioned that you were not charged for the new gamertag, you will need to check the Enforcement history on your account as it could be a forced gamertag change. Any gamertags that violate the code of conduct (it doesn't necessarily have to be offensive) are given a randomly generated tag. That is per the code of conduct you agreed to.

However, if there's no enforcement action taken on your account, the only explanation to this situation is that someone else has access to your account. If you haven't used your free change, that someone used it and changed your old gamertag to the current one. In this case, you will need to change your password and update your security info.