TMZ's obtained video from one concertgoer first showing Monica, bleach blonde hair cascading down her back, belting out "Angel of Mine."

Later, a figure with those same light locks appears passed out at the side of the stage with a member of the crew quickly carrying them away from the spotlight.

And, then there's this ... several fans took to social media and alluded to some kind of incident at Monica's show in The Bayou City.

One X user asked "What happened to Monica at the Houston concert??" while another added, "The Houston streets are talking! They’re saying Monica left in an ambulance after passing out by the stairs."

Monica is no stranger to health issues. She's often talked about her mitral valve prolapse, a heart condition she was diagnosed with at 18 which impedes the flap between the upper and lower chambers of her heart from closing properly.

She's also suffered from, and been hospitalized for, high blood pressure in the past, a condition she's said she's worked hard to control, and she's had multiple surgeries for endometriosis.

43-year-old Monica has several shows coming up in the American Southeast on her tour ... perhaps heaping extra stress on the star's plate.