MGK took the stage to perform "Twin Flame" ... telling the crowd before he began singing, "I wrote this song for my wife."

Then he broke into song, and midway through he said, "This is for our unborn child." The line was off the cuff and isn't in the original recording.

Now, here's the mystery ... during the song, as he speaks about their unborn child, you hear a heartbeat under the song. The heartbeat stops, and MGK sings, "Go to sleep, I'll see you in my dreams. This changes everything. Now I have to set you free."

At the end of the performance, the camera pans to Megan in the audience, and she's in tears.

There's a debate online as to whether MGK is referring to a miscarriage.