KARRUECHE TRAN: ROUGH DAY IN CHICAGO.

Karrueche Tran's bloody lip and vague caption might have you believing something seriously physical went down a certain someone in her life ... that's not the case.

KT posted the photo Tuesday afternoon with the caption, "Rough day in Chicago." While it seems like a bad domestic violence joke -- we're told she was actually referring to a photo shoot she just did in Chi-town.

Our sources say the spread called for Karrueche to play a "street fighter" type, thus the blood.

So, despite her ex-bf temporarily overreacting to her pic with Tyson Beckford -- this fight club snapshot was strictly a work thing.