Tom Sandoval and Ariana Madix seemed to be very much a couple just days before TMZ broke the story of their split … and we’ve now learned more details about what specifically led to the couple’s sudden end.

Sources close to the former pair tell us it was a series of text messages Ariana discovered Wednesday night between Tom and Raquel Leviss that caused her to believe he was cheating. We’re told the messages and video were “sexual in nature” … leading to a confrontation between Ariana and Tom.

Our sources tell us from there, Ariana told friends she found out Tom and Raquel had actually been seeing each other secretly for months — as far back as the summer.

We’re told there was growing suspicion between the former couple’s friends that something may have been going on between Sandoval and Raquel … but it was totally a surprise to Ariana.

As we told you, Ariana was spotted supporting Tom Wednesday night while he performed with his band and debuted a new single. In fact, eyewitnesses at the show say they were kissing and having a good time together — very much still a couple.

Apparently, Tom has a different version … a source close to Tom told us things had been unraveling between him and Ariana for quite some time … but gotta say, that wasn’t evidenced Wednesday before she’d discovered the text messages.

Tom and Ariana got together back in 2013 when she joined “Vanderpump Rules” as a full-time cast member … while they never got married, they were certainly one of the anchor couples on the reality show for years.

Neither Tom or Ariana has commented publicly on the allegations … reps for both gave us a “no comment.”