Kim Kardashian and Thomas CaldwellKourtney Kardashian Barker's relationship may be hot and cold, but there is a good reason why the eldest Kardashian sister missed Kim's birthday.

Kim celebrated her 43rd birthday on Oct. 21 with a party over the weekend. The whole gang was there, including Khloé Kardashian, Kris Jenner, Kylie Jenner and Kendall Jenner.

A host of friends were also in attendance, including Hailey Bieber, Sara Foster and Ivanka Trump. And while Kourtney, 44, was noticeably absent, her younger sister was quick to explain where she was in an Instagram post.

"So blessed to have hit the jackpot of friends. I couldn’t have ever dreamed I would be so lucky to call these girls my friends. Thank you for all the birthday love," Kim captioned the post. "And Kourt I’m jumping in bed with you next week for our bed rest picnic."

Kourtney is currently pregnant with the baby she shares with husband Travis Barker. She was hospitalized last month after undergoing "urgent fetal surgery."

"As someone who has had three really easy pregnancies in the past, I wasn’t prepared for the fear of rushing into urgent fetal surgery," the "Poosh" founder wrote in an Instagram post at the time. "I don’t think anyone who hasn’t been through a similar situation can begin to understand that feeling of fear."

Kourtney shares kids Mason, 13, Penelope, 11, and Reign, 8, with ex Scott Disick.

In Season 2 of "The Kardashians," the couple went through in vitro fertilization treatment and talked openly about trying to have a baby together. The next season, Kardashian revealed they had stopped trying IVF, opting to let pregnancy happen naturally.

Kourtney's absence from the Skims founder's birthday party comes amid a growing feud, as detailed on the family's reality show on Hulu.

From a secret group chat without Kourtney to Kim being labeled a narcissist and bringing up how Kourtney's kids come to her "with problems," their latest feud "got to a place that (they) weren't proud of."

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Kourtney said on the show that her family has a tendency to try to hurt each other with their words, and it’s something she's trying to work on herself in therapy.

"When I'm reminded of those types of things, it really is hurtful," she says. She finds herself thinking, "'Why would my family treat me that way?' And then I want to protect my energy and be around positivity and good vibes, and Palm Springs with my husband is where it's at."

Contributing: KiMi Robinson and Erin Jensen

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