'Vanderpump Rules' Drama is a SUR Thing Tonight

"Vanderpump Rules" star Jax Taylor's got some serious girl trouble -- as in, his gf might have cheated on him ... with a girl.

Brittany Cartwright insists the girl-on-girl action never went down, but Jax ain't buying it just yet ... which has the couple on the verge of an all-out war in season 5.

The showdown has all the ear markings of becoming the latest in a long line of OMG moments on the show. Here's a few to jog your memory:

- Jax inking himself up with his gf Stassi's name ... which was sweet, until he moved on to Carmen. Yeah, he got her name tatted too.
- Scheana and Shay finally making it to the altar! BTW, who else gets married in a bejeweled crop top?
- Tom Schwartz making good on the infamous promise "ring on a string" and proposing to Katie.
- New chick LaLa's classy choice to go topless in Hawaii ... instantly pissing off Katie.

With this crew, no one EVER checks their drama at the door.

"Vanderpump Rules" airs tonight at 9/8c on Bravo.