MGK performed Saturday at the Carolina Rebellion music festival in NC, and was obviously still suffering from his busted sternum. He repeatedly clutched at his chest during his set, but powered through it to keep up with his band. The pounding bass from the speakers can't help.

MGK's rep tells TMZ ... even though he's hurting, Kelly doesn't want to put off any more shows than he already has -- and he's doing his best to heal in his downtime.

We broke the story ... Kelly's recovering after filming a new movie, "Captive State" where he got punched 65 times.