Willie Nelson worried his fans when he abruptly canceled his tour due to a "breathing problem," but the singer's only taking a temporary break and will return soon ... TMZ has learned.

Willie's rep tells us his tour schedule is typically 2 weeks on, 2 weeks off, but his recent medical issue -- which they believe is linked to his history of emphysema -- is requiring a little extra R&R this time.

We're told he's only officially canceled the next 6 stops on his tour ... but plans to return to the stage September 6 in Gilford, NH and continue on with the 20+ shows scheduled after that.

As for the breathing problem ... Nelson's rep says it was not a medical emergency and he wasn't rushed to the hospital or anything like that. He just needed rest and is back in Texas to see a doctor.

We're told he's currently taking it easy and already feeling better.

Willie's 86, though, and this isn't the first time he's had to cancel some shows due to his health. He missed the Outlaw Music Festival last summer and had to cancel some dates earlier in 2018.