Oona Chaplin was originally announced as playing 'Jeyne'. A later website interview seemed to confirm this was Jeyne Westerling. Later on, a rumour from the set suggested that she was actually Jeyne Lannister (or possibly Hill) and would play a role in Jaime's failed escape attempt (which in the book was done with some agents Tyrion sent), although other on-set sources didn't seem to back this up.

Based on all of this, two possibilities remain:

1) The girl is as she appears and the earlier references to Jeyne were merely referring to Oona Chaplin's character's plot function (i.e. Robb Stark's love interest) rather than her being the literal character. Robb falls in love with her and marries her despite her being a commoner, which is what causes the rift with the Freys (and making it even worse).

2) The girl actually is Jeyne Westerling. in the books, Jeyne's mother is from the Free Cities (not Volantis that I remember though) and she could well have a different appearance because of that. She's joined the Lannister war effort as a healer/battlefield nurse as a volunteer (which may actually be something that's normal in Westeros, or it may be down to her wilfulness; she may even have sneaked off under an alias).

A more Machivellian possibility is that she's deliberately been planted to catch Robb's eye, but that seems highly implausible, given the Starks staged a surprise attack and secured a totally unpredictable victory that could not have been foreseen.