Merrianne Do achieved overnight fame for her animated expressions while supporting Justin Herbert and the Chargers during their 20-17 loss to the Dallas Cowboys at SoFi Stadium ... and many were quick to accuse her of being planted by the team or league for manufactured clout.

TMZ Sports spoke with the internet's newest celebrity ... and she says the truth is, she's your average fanatic who's super passionate about her squad.

"I wish I was being paid," Do tells us. "If [the skeptics] only knew this is how I am every day. My kids are like, 'Mommy this is how you are at my flag football game, why do they think you're getting paid?' I'm like. 'I know, but they don't know me.'"

As it turns out, it's true -- Do and her husband are truly diehard Charger fans -- the two have season tickets in the second row ... and if they aren't sittin' there, they're posted up in the cabana suites.

"We are diehard fans," she says. "It's always been like this. People that know me know that I've been this crazy football fan all my life."

The internet sleuths have been quick to try and poke holes in Do's explanation ... even posting pics of her in Minnesota Vikings gear -- but she has a reason for that, too.

"I grew up in Minnesota before moving to California almost 20 years ago," Do says. "I don't think you have to be a single-team fan. I love my Chargers and I'm not gonna deny my Vikings. At the end of the day, yes, I've been a Vikings fan, I've been through that journey as a Vikings fan and still am, but I'm here in LA for the past 20 years."

Long story short, Do didn't get a cent for her viral moment ... but knowing how internet fame works, that could all change.

"To anybody that thinks this is paid, I am not paid, I'm still eating my ramen noodles."