'VPR' Brittany Cartwright Details Jax Taylor Fight That Led to Separation

Brittany Cartwright is dishing the details about the showdown with her estranged hubby, Jax Taylor, that was the last nail in the coffin for their relationship.

The "Vanderpump Rules" star talked about the pillow talk gone wrong Tuesday night on "Watch What Happens Live" ... revealing Jax was pissed off she went on a night out with her "The Valley" costar Kristen Doute, and his attitude became Britt's breaking point.

BC reflected Jax had "kind of made up a story in his own head and started a fight about something that never happened" -- and it was then she realized everything in their marriage was wrong.

It wasn't just that incident -- Brittany said their marriage turned into a full-blown battleground where they fought over every little thing, and she just wanted to remove herself from the situation.

Brittany also debunked those cheating rumors once and for all, echoing what our sources previously told us of there being no infidelity between them.

As for where they stand right now ... they're still separated, and she's still crashing in an Airbnb.

You'll recall, the reality stars announced their separation in Feb on their joint podcast, "When Reality Hits with Jax and Brittany" ... with BC saying she needed space for her own mental well-being and for the sake of their 2-year-old son, Cruz.