We spoke with multiple sources in the USC athletic department who all say the same thing ... the player who confronted Shaw -- a respected member of the defense -- was pissed off because he knew the story was bogus and got right in Shaw's face about it.
Josh Shaw was aggressively confronted by a teammate in the USC locker room Monday morning -- with the player screaming at Shaw to "come clean" about the false hero story ... TMZ Sports has learned.
During the confrontation -- which occurred right after the initial story about Shaw saving his 7-year-old nephew from drowning was posted on a USC media outlet -- the player yelled at Shaw, "Why you lying?"
We're told the situation got very intense very quickly -- and several teammates rushed in to separate the two before things got violent.
As we previously reported, Shaw eventually fessed up and admitted he made the whole hero story up -- but he has yet to come clean about why he concocted the tale in the first place.