Getting back to an account

Let me explain my situation.

I have a fixed username for every game. Today I downloaded Call of Duty (I have played it in the past as well), and turns out an account with the same username as mine already exists. Every letter, case, spacing is the exact same.

Now, I am confident it is one of my old accounts, because in the match history, a game has been played where one of the teammates is my cousin.

But unfortunately, I have checked all the accounts I could, but none of those link to that username.

What can I do in this case?

Ps - I bought a new phone, so it could be the guest account of my old phone.. But my heart and my mind tells me I would NEVER make a guest account with my unique username

Can anyone please Help?