Pink fans in the Midwest are sure to be a little disappointed ... 'cause she just postponed a few shows out there -- a decision she says it out of her hands.

The singer-songwriter posted to Instagram Saturday morning ... telling ticketholders in Nebraska, South Dakota, Wisconsin and Iowa she's gotta put off the shows she planned in their states.

Pink doesn't explain why she's can't perform on the scheduled dates ... chalking it up to "reasons beyond my control."

No new dates for fans who had tickets yet ... but, Pink says she plans to release the rescheduled concert dates soon -- before thanking everyone for their understanding.

Pink is in the middle of her "Summer Carnival" tour ... and, because she's not performing in these next four shows, she's now got a long hiatus. Her next show is scheduled for November 3 in Austin, Texas.

The singer posted as recently as yesterday by the way, sharing pics and videos of Graham Cracker ... the dog she's fostering while on tour in hopes of finding him a permanent home.

We've reached out to Pink's people about what prompted the cancelation ... so far, no word back.