July 20, 2011  -- An insider who once worked with Rep. Michele Bachmann (R- Minn.) told ABC News he believes her migraine headaches have been a persistent and vexing problem for years, often forcing the congresswoman to cancel meetings, shut her office, turn off the lights and lie down until they pass.

"You're probably talking once every two to four weeks where she would have severe headaches, and at times it would be more frequent," the former insider told ABC News, speaking on the condition he not be identified because he was concerned about retribution. "They would come on in a matter of minutes. She would be down on her couch with the lights off and unable to function."

The GOP presidential candidate confirmed to reporters Tuesday that she suffers from migraine headaches, but she said they were controlled by medication and her condition would not affect her ability to serve as president.

"I've maintained a full schedule between my duties as a Congresswoman and a presidential candidate traveling across this nation," Bachmann said. "I have prescribed medication I take on occasion whenever symptoms arise and they keep my migraines under control. But I'd like to make it abundantly clear, my ability to function effectively will not affect my ability to serve as Commander in Chief."

Bachmann declined to answer a question from ABC News about whether she had ever had to skip Congressional votes because of migraines. Her security staff blocked reporters when they tried to follow her and ask her further questions.

Bachmann made the statement in response to a widespread report by The Daily Caller which alleged the Minnesota congresswoman's migraines could "incapacitate" her "for days at a time."

"It absolutely does not affect her ability to do her job. And these are difficult, long, stressful hot days, so it is bogus to say that she's incapacitated," Bachmann spokeswoman Alice Stewart told ABC News.

The former insider offered ABC News a different account, and said he was surprised the campaign was attempting to portray the medical condition as innocuous. He said the headaches occurred so often that Bachmann's scheduler had frequent conversations with the medical offices in the House of Representatives in order to be able to respond quickly with medication.

"We had many occasions where there would be a stressful situation -- they were almost always stressful situations," the Bachmann insider said. "We would have lights out. Close the office, shut the lights and she would lay in the office and try and recover. Everyone from interns on up knew about it."

The Bachmann campaign released a letter Wednesday afternoon from Dr. Brian Monahan, the Attending Physician of the Congress, addressed to Rep. Bachmann, in which the doctor confirms the diagnosis of migraines and says Bachmann is able to control them using medication. "Your migraines occur infrequently and have known trigger factors of which you are aware and know how to avoid," wrote Dr. Monahan. "When you do have a migraine, you are able to control it well with as-needed sumatriptan and odansetron. It has not been necessary for you to take daily scheduled medications to manage this condition."

Neither Bachmann nor Stewart addressed the specific allegation in the Daily Caller story that Bachmann has been hospitalized for the headaches, including one instance on Friday, July 30, 2010.

According to Congressional records, of the 15 votes that took place that day, Bachmann did not participate in nine of them.

"She was incredibly sensitive about missing votes," the source said. "Her opponent was pummeling her on it. For her to miss nine votes that day, she would have had to be out of pocket and unable to function."

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