EXCLUSIVE: Inside Rob Kardashian's Baby Shower Drama, What Led Him to Tweet Kylie Jenner's Phone Number

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When Rob Kardashian is upset about something, he's quick to take it to social media.

After the 29-year-old reality star tweeted out his little sister Kylie Jenner's phone number to his 6.6 million followers on Monday night -- and called out his family for not inviting his pregnant fiancée, Blac Chyna, to a baby shower -- a source exclusively tells ET what happened behind the scenes.

WATCH: Rob Kardashian Thanks Yolanda Hadid for Encouraging Him to Keep Being 'The Real Deal'

"Rob was told that the shower was still going on for him but Chyna couldn't make it, but he misunderstood the entire thing and overreacted," the source says. "The girls knew Chyna couldn't make it, and Chyna was cool with it, but Rob thought something was being done behind closed doors to make sure she couldn't be there."

Another part of the problem, Rob "is still convinced his family doesn't like Chyna," according to our source. However, that couldn't be further from the truth. Rob's sisters -- Kourtney, Kim, and Khloe Kardashian, as well as his younger sisters, Kendall and Kylie Jenner -- "absolutely adore" their future sister-in-law.

"[They] thank her every day for coming into Rob's life and changing him," the source says. "He's such a better person with Chyna, and all the girls see that."

As for the fact that Chyna was once engaged to 19-year-old Kylie's current boyfriend, rapper Tyga, and has a 3-year-old son with him, the source adds, "The entire situation is so bizarre, and Rob has always been insecure about that."

According to the source, Rob and Chyna's relationship has been strained in recent days, though she's not planning to leave him. Viewers of the couple's E! reality show, Rob & Chyna, have already gotten a peek into their sometimes explosive arguments.

"Rob and Chyna haven't been doing well lately," the source says. "Rob is in one of his funks right now."

Last month, a source close to Rob told ET that "mood swings" were common for the formerly reclusive reality star, after he deleted all of his Instagram posts from his official account -- including snaps of Chyna.

"He goes through his dark periods," the source said. "He gets depressed and doesn't want to really do anything, doesn't leave the house. It got better for a bit when he first started dating Chyna, but he's in a rut now."

WATCH: Rob Kardashian Meets Blac Chyna's Extended Family, Gets Words of Warning From Her Mom

Though last week, he showed off his more playful side, teasing his five sisters for all going blonde at one point by Photoshopping a picture of himself with his own sleek blond locks.

"Pretty sure I murdered this look… who you think wore it best?" he joked on Instagram.

Watch below:

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