Hey Aalzuko,
Sorry to hear someone else might've gotten into your account. If you haven't already, please be sure to go through as many of these security steps as possible to secure your account just to be safe.
Before contacting live support to regain control of your account, please double-check if the EA account you're currently signed into is the one you used to have Steam linked to in the past. Since game progress is stored at the EA account level, and there are cooldown periods between relinking accounts, you'll want to be sure to avoid being stuck with the wrong account. You can confirm by checking your order history for past purchases, reviewing email addresses for past communication from EA, and logging in to the EA App to see if your time played and library are accurate.
After that, please get in touch with our support team to help recover the account and correct the linking. The team will need to discuss private account details, and we can't support account concerns here on the forums.
Let us know how it goes, thanks!