More trouble for NASCAR -- Kyle Busch is apologizing for using the r-word in a post-race interview on Sunday ... saying he should have never used the derogatory term.
Busch was furious with Brad Keselowski after making contact with the driver at the NASCAR Cup playoff race at Martinsville ... sounding off to reporters following his 2nd-place finish.
"He drills my ass coming out of (turn) 4 for no reason," Busch said after the race.
"I mean, where was he going?