Karrueche Tran -- Oh, That Bloody Lip? It's Just ... (PHOTO)

Karrueche Tran's bloody lip and vague caption might have you believing something seriously physical went down a certain someone in her life ... but TMZ has learned that just isn't 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.