It wasn't the birthday blowout Rob had expected ...

Pattinson and his on-again-off-again (on-again) girlfriend Kristen Stewart didn't celebrate the actor's 27th birthday together, they spent it in a heart-wrenching fight!

X17 photographers spotted Kristen at a friend's house Monday afternoon and the actress was in break-down mode over what appeared to be a possible break-up with Rob.

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

After KStew finally wrapped up the heated conversation, she and her pal drove to another friend's house and stayed outside in the street, in her friend's car, talking for almost 45 minutes.

Seems like a tense time for the pair. They celebrated Kristen's birthday just last month, partying with pals and all smiles. What happened? ... Could there be trouble in Twilight paradise?