"It was five days late, it's a Friday, so yeah [I'm surprised]," Nelson said Friday. "I don't [agree with the fine]."
The viral video was actually a combination of multiple plays, a team spokesman said earlier this week.
The Colts had taken the audio of Nelson screaming from another play and added it to the play where the No. 6 pick in this year's draft pulled and flattened Jacksonville Jaguars safety Barry Church into the ground on a Marlon Mack run. The scream came from a play where Nelson was acting as a decoy on an Andrew Luck bootleg.
"I saw it got pretty viral on the internet, which was cool," Nelson said Tuesday. "... Yeah, I wasn't yelling, not on that play. I don't know how it got amped up like that."
The team has since taken down the tweet.
A source said Nelson is appealing the fine from the play, on which no penalty was called.
ESPN's Mike Wells contributed to this report.