'Big Brother' Contestant Jeff -- Dude, You Can't ... Y'Know ... Masturbate On Your Housemates!! (UPDATED VIDEO)

"Big Brother" might have a sticky situation ... after houseguest Jeff appeared to masturbate, and wipe a fluid on houseguest Julia's back.

The incident was caught on the live feed Thursday night -- Jeff and Julia were lying in bed ... he seemed to be pleasuring himself under the covers while talking to her, and then joked about putting "it" on her.

Hard to tell if Jeff really did anything nasty, since it's all under covers -- and it seems unlikely (based on time alone) -- but viewers believe the jerk was real. Social media is blowing up with people pissed that CBS let this go down ... or on.

It's unclear if this will be addressed on Sunday night's broadcast ... but you gotta figure.

We've called CBS for comment. No word back.

9:45 AM PT -- A new camera angle's surfaced ... and this one appears to clear Jeff's name. Looks like there really was a stain on Julia's shoulder prior to him touching her. Also, you can't see that jerking action, but that could be because the camera's further away.