Britney is responding to the story TMZ broke early Thursday ... that the head of Victor's security backhanded her in the face outside Catch restaurant in Vegas. The account she just posted on social media aligns with what she told the police.

Britney says, "I recognized an athlete in my hotel lobby as I was heading to dinner. I later went to a restaurant at a different hotel and saw him again." That hotel is Aria.

She goes on to describe the encounter ... "I decided to approach him and congratulate him on his success.  It was really loud, so I tapped him on the shoulder to get his attention."

Now the dispute ... "I am aware the players statement where he mentions 'I grabbed him from behind' but I simply tapped him on the shoulder."

As for the slap, she says "His security then back handed me in the face without looking back, in front of the crowd. Nearly knocking me down and causing my glasses off my face."

Britney adds she herself was "swarmed" by around 20 fans on her way into Aria, noting, "My security team didn't hit any of them."

As we reported, Victor spoke with reporters -- including KENS 5 reporter Nate Ryan -- after practice Thursday to detail his side of how things went down.

Britney says, "“I also don’t appreciate nor do I thin this is a laughing matter. Watching the player smile and laugh was cruel and demoralizing with the situation that took place. I am 5’3 and he’s 7’4…”

Sources close to Britney tell us she hasn't yet heard from Victor's team and she's pissed about it ... thinking someone would've at least reached out to apologize.

TMZ spoke to an eyewitness who says he not only saw the head of security slap Britney, he heard the impact.  He also says Britney was furious, screaming "This is f****** America."

As we reported, Metro PD has launched a criminal investigation and interviewed Britney this morning at her hotel. We're told the case will "likely" be referred to the D.A. for review.