Alright, that's what I thought.
I'm betting that after Eggman found the Phantom Ruby, he tortured Sonic and experimented on him with it. The result was creating another "Ultimate Lifeform" / Sonic clone, this time powered by the Phantom Ruby, known as Infinite, that's been brainwashed to be loyal to only Eggman.
I mean, his theme kind of backs that up:
"Cause I've been up against better / Just take a look at my face"
"Cause I was born in this pain / It only hurts if you let it"
"This pain persists, I can't resist / But that's what it takes to be Infinite"
Watch as this is actually right.