Either you were an unregistered user when you were posting questions in 2015, or you used a different email address / login method on that original account. Given that this is your old account, and that it appears to be a registered account, it sounds like it's the latter case.
If you were unregistered, you may no longer be able to access that old account, since it was tied to browser cookies. If it's just a matter of a different email address, which it appears to be, then you'd have to use that same address to log in to it again.
However, in any case, you can request that your old "account" be merged into your new account, then your old questions/answers/etc. will be tied to your newly created account.