Aldon Smith -- On-Field Fight with Colin Kaepernick

Aldon Smith got into a scrap with 49ers star Colin Kaepernick earlier this week ... which may have been fueled by ongoing rumors about Smith's ex-girlfriend and the QB.

Multiple Niner locker room sources tell TMZ Sports ... Aldon and Colin had an altercation on the field during a practice on Tuesday, and "players had to pull them apart." It's interesting because there's been a lot of Internet buzz Smith's ex-gf is now with Kaep.

Smith and Hot 97 host Nessa broke up in recent months, and even though she and Kaep have not been photographed together ... rumors they're together have persisted. Our sources say it wasn't clear if Smith and Kaep exchanged any words before the fight broke out ... or if Nessa's name ever came up.