Uh oh! Rob Kardashian is under criminal investigation for allegedly threatening singer Pilot Jones, who has been getting pretty chummy with Blac Chyna.

Rob allegedly physically threatened Pilot because the wannabe singer's been hanging with Rob's lady, according to law enforcement sources in a new TMZ report.

The threats come right after photos surfaced of Chyna and Pilot making out -- although it's unclear if the photos are from before she started dating Rob or during a break ... or if she's cheating.

It's believed the pregnant former stripper dated Pilot before she hooked up with Rob and rekindled a romance with Jones during one of her breaks from the overweight Kardashian. Back in March, Chyna partied with Pilot at a club in Atlanta -- and they looked mighty friendly. There have even been rumors that Pilot is the true father of Chyna's unborn. Talk about DRAMA RAMA!

The outlet is also reporting that Pilot is scared because of the string of texts Rob him sent, like, "Leave my wife alone. I'm not playing with you."

That doesn't sound like a threat to us -- but apparently there are also texts in which Rob generally threatens Pilot with physical harm if he doesn't steer clear from his baby mama, but a source say that the threat is "a little murky and general."

Sounds like Pilot is kind of acting like a baby! Don't mess with a dude's baby mama and this kind of stuff won't happen!

While the threats may not be that serious, LAPD's Threat Management Unit is investigating the situation. Eek! That sounds serious to us!

Meanwhile when Rob isn't threatening his fiancee's possible lover, he's prepping for his daughter, who is due next month. It appears as if Rob and Chyna have actually been preparing two separate nurseries for baby girl Kardashian -- one at his house and one at hers. That doesn't look good for the twosome's future as a couple if you ask us!

An insider told People, “They have separate nurseries because they both wanted to prepare for the baby, but they agreed it wasn’t a good idea to live together while Chyna is pregnant because they were fighting too much." Ain't that the truth?!

According to the source, the couple has yet to pick with house they will live in together after the baby is born but “their daughter will obviously stay with Chyna.”

Newflash: that drama-filled duo ain't ever living together as a couple!

UPDATE: Chyna said that the photo in question was taken over two years ago. She vehemently says she's never slept with Pilot and all of this is a plea for attention to get press for his new track. The reality star went off on Instagram with an expletive-filled rant, in which she posted several conversations with Pilot (whose real name appears to be Justin) saying that Pilot is gay and trying to extort money from her, writing, "Ok let's state facts !!! I been quiet for too long!!! So JC Pilot Jones whatever the f--k his name is has already tried to put out a story saying that he's the potential father of my child really ?!?!?!"

The passionate lady added, "I never had sex with him ! NEVER EVER! Nothing ever happened past these pics."

That's messed-up stuff! We gotta say -- we are definitely believing Chyna on this one!