|Robert Pattinson & Kristen Stewart|
The stars recently ended their four-year on/off relationship and while the pair were seen as inseparable before becoming distant after the 'Bel Ami' actor returned to Los Angeles from New York earlier this month.
An insider told People magazine: "They again went from spending every second together, to spending more time apart at their separate houses.
"Kristen has been in a terrible mood, and it was obvious that things were not great with Rob."
It is said Robert's 27th birthday on May 13 was a hint at the downfall of their romance with the actress not present on his big day.
The source added: ""It was very strange that they didn't celebrate Rob's birthday together.
"He had friends over at his house, but Kristen was not part of the celebration. For the past few days, they have not spent any time together."
The pair have had their problems since Kristen, 23, had a fling with her 'Snow White and the Huntsman' director Rupert Sanders.
They split up over the affair for two months last summer, but the former 'Twilight Saga' stars were said to be closer after Robert returned from a two month stint in Australia in March, where he was busy filming his upcoming movie 'The Rover'.