Benji Cheats on Paris?!?

Paris is not going to think this is hot at all!

According to Star magazine, it seems Paris' BF Benji Madden has apparently been missing his ex-fiance.

The Star claims their spies caught the Good Charlotte rocker getting rather personal with a girl who was a dead ringer for Sophie Monk at the Diesel xXx Rock & Roll Circus concert in NYC.

"He seemed really into her. She kissed him on the cheek, and he was hugging her in a corner of the VIP section, trying not to be seen," the report claims.

BUT HERE'S THE CLINCHER ... the problem could be more than just missing Sophie ... The mag says Benji is just fed up with Paris, and that he's reportedly even called her "shallow and dumb" around friends. Dayum!!!

The mag says Paris taking this hard; they claim Paris' pals say she's miserable all the time and crying about Benji's infidelities.

Think there's any truth to this?? Benji just doesn't seem that mean!