Aubrey O’Day had a recent medical scare … and that’s the reason she had to skip the Danity Kane reunion show Wednesday in Los Angeles, a stop on the group’s comeback tour.

On her Instagram Story, Aubrey explained she was hospitalized after feeling sick all day … fighting her undisclosed symptoms until she finally had to go to the ER — saying she was “heartbroken” to let fans down.

Aundrea Fimbres and D. Woods ended up performing as a duo at the El Rey Theatre … the second stop on Danity Kane’s “The Untold Chapter” tour, which was announced in October.

Aubrey has been in the news lately because she appeared in 50 Cent’s Diddy documentary on Netflix … reading a sexually charged email allegedly from Diddy … and said she believes she was fired from the “Making the Band 3” group because she refused to sleep with him.

Along with the health scare … the singer posted to IG on Thursday that it had been a “heavy time” for her — and that it was stressful for fans to finally hear in the Netflix doc “things that were hard for me to say.”