Machine Gun Kelly's Broken Sternum Forces Concert Postponements

Machine Gun Kelly is down for the count with broken bones in his chest ... an injury he suffered while shooting a movie last week.

MGK's rep tells TMZ ... he has a contusion and hairline fracture of the sternum (breastbone), and that's why he's had to reschedule the first 3 stops on his tour. Kelly announced the postponements Thursday night on social media, but kept quiet about the injury.

As we reported ... Kelly struggled through a show last weekend in Florida, even walking off stage at one point. Although he finished the show, it was obvious he was in a ton of pain.

Bad timing for the rapper ... his new album, "Bloom," comes out next month ... however, the rep says he's expected to make a full recovery, and Kelly will make up the tour dates.