"THAT'S MY BOYYYYYYYYYYYYY," she yelled from the suite ... adding, "WOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!"
Once it became clear the Chiefs were taking control, Matthews sent a video message to all of her haters ...
"All those people that doubted us and hated on us, that thought we were gonna lose this game? GUESS AGAIN!!!"
Matthews ain't exactly a wallflower at Chiefs games -- she's as loud and rowdy as they come -- which might be why Patriots fans heckled the crap out of her during a regular-season matchup at Gillette Stadium toward the end of the season.
The good news for Brittany ... the Chiefs will be back at Arrowhead next weekend to take on the Tennessee Titans in the AFC Championship game.