Something must have been really funny!

Kendall turned 19-years-old today, and the model celebrated with a birthday lunch at the Hotel Bel-Air with sisters Khloe and Kourtney Kardashian and mom Kris Jenner. But Kimmy K and Kylie skipped the party! It must have been a girls only affair, because Scott Disick, Kanye West, Rob Kardashian, and Bruce Jenner weren't in attendance either.

Our photographer at the hotel tells us, "Kendall didn't look worried about her missing sisters. She and Kourtney and Khloe were having a great time. no one seemed concerned that Kylie and Kim weren't there."

Meanwhile, we spotted the younger Jenner getting hair extensions at a Beverly Hills salon during the time her sisters were celebrating! Hey, priorities ... or is there more going on? ... Could there be a beef between Kendall and Kylie? Or maybe mom Kris is upset that her 17 year-old is dressing like a slut for Halloween and going to an adult party with 24 year-old Tyga? Hmmm ...