Rob Kardashian in a scene from Keeping up with the Kardashians, season 9.

London - Rob Kardashian has admitted he is in a lot of pain.

The 'Keeping Up with the Kardashians' star, who gained 50 pounds following his ugly split from Rita Ora in 2012, posted a cryptic message on Twitter, amid reports he is increasingly unhappy about his weight and battling depression.

The 27-year-old reality TV star tweeted: "No one will ever understand how much it hurts."

His troubling post came just hours after his ex-girlfriend hit the headlines for telling the UK's Elle magazine that she had never "experienced" love before her current boyfriend Calvin Harris.

Rob, who failed to join his family on their holiday to Phuket, Thailand, has made no secret of his desire to get back in shape over the past year.

A source previously told "(He is) really depressed. The family is very concerned about him."

The insider also claimed that despite being open about his desire to lose weight and having a personal trainer on speed-dial, Rob "rarely shows up" to his training sessions.

Another friend recently claimed that Rob doesn't want to attend his older sister Kim Kardashian's wedding to Kanye West in Paris in May because he's so unhappy with his appearance.

The source said: "Rob is upset about his changing shape and truly feels like the black sheep of the family. Kim is his sister. He loves her. But he knows this will be all about Kim and as usual, Rob feels he would be like the third wheel.

"The reality of their situation is that Kim and Rob haven't been that close in years. Deep down, she thinks he is a total slacker and it's very hurtful to Rob." - Bang Showbiz