Jason ran into trouble during his Connecticut concert Saturday night ... he was filmed bolting offstage in the middle of a song, and although we don't know, there are signs it was heat-related.

The country star was performing in the heat and humidity at the Xfinity Theatre in Hartford when he stopped singing and buried his face in his upper arm. It looks like he was wiping off sweat, then he just ran off stage.

Check out this video ... Jason starts belting out the words again while standing at the mic, but it's all very brief, and that was the end of the show. Something is clearly wrong.

We don't know what happened after that ... but one of Jason's fans took to Twitter to provide some details.

She wrote ... "It was hotter than hell. It was bad enough just standing in the pit I can't imagine performing in jeans, boots, and hat under all those lights in that humidity with no breeze. Hope he's okay!"

She also said Jason never returned, and the show was cut short. Jason is currently on his "Highway Desperado" tour, so we hope it's not too serious.

We've reached out to Jason's reps for comment ... so far no word back.