You are Knuckles. You were guarding the master emerald, when Eggman flew up, just out of your reach.

Eggman: "Well if it isn't Knuckles the Echidna, I have a surprise for you."

The leaves behind you rustle, and you turn around to see a mechanical version of you emerging from behind a bush.

Eggman: "Allow me to introduce Metal Knuckles."