CHICAGO – Chicago Teachers Union President Karen Lewis, who is considering a run for mayor, has been hospitalized, a union spokeswoman said Monday.

CTU spokeswoman Stephanie Gadlin says Lewis, 61, is being evaluated at a local hospital “after experiencing discomfort Sunday evening.” Gadlin says it is not due to a stroke, as rumors have suggested.

“We respect Karen’s privacy just as we value her leadership.  I’m sure she’ll determine whether or not another public statement is warranted,” Gadlin said.

Gadlin said Lewis is resting well and in good spirits.

Whether Lewis’s discomfort is related to bariatric surgery performed in Mexico, which preceded her dramatic weight loss, isn’t clear, CBS 2 Chief Correspondent Jay Levine reports.

Just three weeks ago, Lewis seemed fine, attending the Mexican Independence Parade. She told reporters then that a crowded mayoral primary would be  healthy for voters.

Lewis has led the CTU since 2010. She has been a vocal opponent of incumbent Mayor Rahm Emanuel, particularly over last year’s public school closings.

“The Mayor wishes Karen well, and that she makes a full and speedy recovery,” Emanuel’s office said in a prepared statement.

Ward 2 Ald. Bob Fioretti has formally announced he will run.