So here's the story behind the story of these pictures the other day when we snapped Britney crying behind the wheel of her car ...

On Tuesday afternoon, Britney headed to the gym for a good sweaty workout. She was in a great mood when she left, according to an X17 photographer, "singing in the car, fist-pumping through the sunroof as she drove with her bodyguard in the passenger seat."

As the pair headed to the 405 Freeway to go back to her Calabasas home, Brit turned around and headed back to West Hollywood where she met up with Jason at his place.

Jason wanted to bring his car so he followed his girlfriend but at one point he decided to join her in her Mercedes G-Wagon and have her bodyguard drive his car. He called Brit on the phone to ask her to pull over, which she did, but she was blocking traffic and cars behind her started honking. This frustrated her and by the time Jason got to the car, she was ready to explode -- and she did!

Britney flew into a rage and, according to our photographer, "hit Jason in the face five or six times!" Apparently she was slapping him in the face -- our eyewitness says he's never seen anything like it:

    "Britney was really upset. It looked like she was taking out years of frustration on Jason when she was hitting him. It wasn't like this was anger from one incident. It looks like she hates the guy! Why would you do that to someone you love?"

So what does this say about Britney and Jason's relationship? Are these two on the road to trouble? Could Britney be frustrated with the whole thing and want to end it? And even if she doesn't, is it a healthy relationship if Britney's abusing Jason? Seems like something's gotta give ...