Karrueche Tran -- It's Not What You Think With Rob ... Just Oral Surgery

Karrueche Tran and Rob Kardashian are old friends ... and the suggestive photo he posted was just her showing off her swollen face post-dental surgery.

Karrueche went under the drill Tuesday to have her wisdom teeth pulled. We're told Rob checked in on his pal and noticed she was talking funny ... after they hung up, she sent him the post-dentist pic.

Our source says Karrueche isn't mad Rob posted the photo and suggestive caption ... they've been friends since high school and regularly talk.

Going to the dentist ... never fun for soooo many reasons.