The Colorado mom who says she does not remember abandoning her two young sons in a van and walking for 12 miles says her son later told her,  "Mommy, you lost me."

Sarah Hatfield is now out of the Colorado hospital where she had been treated since Saturday, undergoing a series of tests by doctors looking for an explanation into what caused 12 hours of her life to vanish.

"I don't know what happened," Hatfield, 26, told " Good Morning America."  "There's just so many unknowns."

Hatfield says the last memory she has is sitting in her van with her two young sons, ages 2 and 4, at a Thornton, Colo., gas station Saturday.  Nearly 12 hours later, around midnight, she arrived outside the National Jewish Hospital in Denver, appearing disoriented as she asked a security guard to use a phone to call home, according to police.

"She called me and said, 'I don't know how I got here, but I'm here. Please come get me,'" Hatfield's husband Matthew told ABCNews.com. "She was frantic and crying and sobbing and just confused. We just have no idea what happened."

Police found the two boys, as well as Hatfield's wallet, cell phone and keys in her abandoned gold van in the gas station parking lot after responding to a call. They say her husband also reported a handgun missing from the family's home.

Police arrested and charged her with two counts of misdemeanor child abuse for abandoning her children.

Hatfield had no injuries when she arrived at National Jewish Hospital, but she was sore, possibly from walking 12 miles down Interstate 25.

"My arms, my legs, my stomach - everything was sore," Hatfield told ABC News affiliate 7NEWS after her release from Adams County Jail Sunday afternoon. She was immediately admitted to Denver Health, a nearby hospital.

Hatfield, who has a history of insomnia and debilitating migraines, told "GMA" today her doctors believe her story, but still do not have a diagnosis.

She said her doctors speculate she could have suffered an epileptic seizure, may have had a psychotic break or experienced a rare but frightening episode of a condition called transient global amnesia, which has been linked to migraines.

After meeting with police and Child Protective Services, Hatfield saw her sons for the first time Wednesday night.  She will continue to only be allowed to have supervised contact.

"Jamie said that I 'lost him,' Hatfield told "GMA" of her son's memory of the incident.  "That's all he would say, 'Mommy, you lost me.'"

ABC News' Katie Moisse and Katie Kindelan contributed to this report.