Brandy says a medical incident drove her offstage in Chicago Saturday night ... revealing a sudden wave of dehydration threatened to make her faint.

The singer-songwriter explained her sudden departure from her United Center show in an Instagram post Sunday morning ... telling her fans that -- after weeks of nonstop rehearsals -- she experienced feelings of dehydration during the show.

Brandy says she did return to the stage after initially leaving it ... but adds she couldn't connect to the tunes sonically -- and acknowledges it wasn't her best show by any stretch of imagination.

After leaving the show, Brandy says she went to a local doctor ... and she's taking proper precautions to avoid a situation like this in the future.

Brandy adds she's deeply grateful to Monica for stepping up in a tough situation ... as well as the entire crew for their support.

We shared video with you of Brandy bailing from the stage last night ... and, at first glance, it seemed she was frustrated with sounds issues -- which makes total sense now given she herself said she couldn't quite connect with the music the right way.

Glad to hear you're doing alright, Brandy ... hopefully the rest of the reunion tour goes off without a hitch!