Sources familiar with the situation told ... just 3 weeks ago, 54-year-old Travis completed his court-ordered rehab stemming from a 2012 drunk driving arrest ... and was finally back on the road.
We're told ... once Travis began touring, he came down with what he thought was a cold ... but turned out to be pneumonia.
Our sources say Travis' health deteriorated quickly ... and Sunday he was airlifted to a hospital, where he was treated for complications stemming from viral cardiomyopathy.
One medical expert told viral cardiomyopathy is not necessarily a death sentence ... and some patients can survive, depending on the level of permanent damage to the heart muscle.
Travis' camp hasn't released the particulars about his medical situation -- only to say he's in critical condition.
Still, we're told Randy's friends and family are "frantic."
Get Well Soon....