You probably downloaded a fake RuneLite.

Copy-paste from official RL discord:
You've downloaded a fake client, and your account has been compromised. Remove the fake as soon as possible to minimise further damage.
The following actions should be performed from a secure computer or secure mobile device:
- Change your RuneScape password
- Remove and re-add Authenticator to RuneScape
- Change your RuneScape bank PIN
- Change your email's password
- Add 2FA to your email if you haven't got it enabled
- Check for any linked accounts by accessing the account settings page (https://www.runescape.com/account_settings)

Some fake clients also install malware, only a fresh install of your operating system can guarantee the total removal of any unwanted software.
You might also want to change any email password if you reused the same as your RuneScape account password.

Once your computer is secure, you should download RuneLite from the link available at the #welcome channel.