The exes, who were inseparable just weeks ago, grew distant since Pattinson returned from New York to Los Angeles on May 8.
"They again went from spending every second together, to spending more time apart at their separate houses," a source tells PEOPLE. "Kristen has been in a terrible mood, and it was obvious that things were not great with Rob."
A major sign pointing to the demise of their romance was the absence of Stewart, 23, on May 13, the date of Pattinson's 27th birthday.
"It was very strange that they didn't celebrate Rob's birthday together," the source says. "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."
Reacting the same as when they last split after Stewart's fling with her Snow White and the Huntsman director Rupert Sanders, the source says the actress once again "[hasn't left] her house for days, but [has] surrounded herself with friends 24/7."