Robert Pattinson and Kristen Stewart have been (somewhat) laying low lately, and have seemingly been avoiding relationship drama, but it looks like that phase has fizzled out pretty quickly. The couple reportedly had a major blowout recently -- and it coincided with Rob's 27th birthday on Monday! (Maybe he was just too broody.)

In fact, Kristen apparently spent Monday with friends rather than with her man because of the fight, although she was spotted and overheard having a long conversation on her cell phone (with Rob seemingly on the other end).

"Kristen was pacing back-and-forth in her friend's apartment, smoking," a photographer reveals. "She looked really upset, talking on the phone for almost an hour."

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The photog adds, "I'm guessing she was talking to Rob. She was really animated, like she was trying to explain something. She was getting frustrated and seemed very emotional."

Well, whatever K.Stew had to explain, it couldn't have been harder or more emotional than filling Rob in on the fact that she cheated on him with her Snow White and the Huntsman director Rupert Sanders last summer. Rob infamously forgave Kristen for her affair, and the two have remained a couple since, but could this latest spat be the end for Robsten?

Something tells us no, considering the crazy obstacles they've already overcome with such a highly publicized relationship. Sources add to the story that after Kristen's long phone convo, she and a friend drove to another pal's house, where they sat outside for 45 minutes talking in the car. A vent session, perhaps? Let's hope it worked!

Do you think Robsten is done for good?

Photos Courtesy of: Fame/Flynet