Kris Humphries -- Friends & Family PISSED He Lied About Myla

Kris Humphries' closest friends and family were BLINDSIDED by TMZ's footage of the NBA player hanging out with his ex-girlfriend -- telling us, they now believe Kris is in a full-blown relationship with Myla Sinanaj ... and has been lying about it for months.

If you recall, Kris had sicked the FBI on Myla in the past ... telling the feds she tried to extort him by demanding a large sum of money, or else she'd release damaging information about their relationship to the media.

Kris told friends he was totally done with Myla ... insisting she was nothing more than a one night stand. Now his friends and family know ... he was lying through his pearly white teeth.

Sources connected to Kris' inner-circle tells us ... they had long-suspected he was secretly getting back together with Myla, but the video of the two leaving her home in Jersey together this week is the smoking gun they were waiting for.

When his inner circle confronted Kris about the TMZ footage, we're told Kris refused to answer any questions ... and changed the subject.

As for Kris' extortion allegations, Myla denied the claim ... and her lawyer even drafted a 26-page defamation lawsuit against the NBA star.