Are you using the exact same login as you do for your members account? Compare the display name. Is the display name of your members account exactly the same as the display name of your account that logs into OSRS?
The only reason I can think why this might happen is if you are trying to access a different account (which isn't a member). This might happen if you are using mobile to access OSRS. You might be using a Game Centre account which won't be the same as your login/pw RS3 account.
The same login/pw will get you access to both RS3 and OSRS and, while your character will be different in each game, the visible display name should be exactly the same.
If you had to pick a new display name to access OSRS, then you accessed a different (possibly brand new) account.
I recommend you use the Forgotten login support page. Two different display names means two different accounts and the tools on the forgotten login page will help you identify the logins you have used.