Chicago Teachers Union president Karen Lewis was rushed into emergency surgery Wednesday at Northwestern Memorial Hospital.
The potential mayoral candidate was taken to the hospital Sunday night via ambulance after "experiencing discomfort." She has remained there since.
A source tells NBC 5 that Lewis, 61, was admitted to the hospital with what appeared to be stroke-like symptoms, but doctors have ruled out a stroke.
According to the source, the surgery is related to a new medical condition -- not related to her heart, or bariatric weight loss surgery she underwent in Mexico in March.
Lewis has lost more than 100 pounds since the surgery, which was performed out of the country because her health insurance wouldn't cover it.
Lewis has set up an exploratory committee and recently started to raise campaign money to potentially challenge Rahm Emanuel in February's mayoral election, but a decision has not been made on whether she will run.
The Chicago Teachers Union announced late Wednesday that it will hold a news conference Thursday to provide updates on the health and hospitalization of Lewis.