Sheryl Stiles suffered the heart attack and fell down an escalator, the report said. A family member said she was on her way back to her hotel room.
Her son, a father or two and 11-year police veteran, was among the 58 people killed Oct. 1 in the worst mass shooting in modern U.S. history.
He was buried Friday following private graveside services. His burial came after a funeral that drew more than 2,000 family members, friends and police and military officers, with honors that included a motorcade down the Las Vegas Strip.
Stiles reportedly suffered a brain injury as a result of the fall and was placed on life support. She died two days later from pulmonary arrest and acute cardio infraction.
Stiles' cousin Cecil Ralston was reportedly told by the police chaplain that Stiles wanted to be with Hartfield and that she "wanted to die."
A year before his death, Hartfield began detailing instructions for his funeral on his computer.
"If you're reading this, then I've been called home," Hartfield's note read. The file was found by his wife, Veronica, following his death.
"Nothing I type will make this any easier, so I will get to the facts. My largest request: Please do not allow anyone to wear black. Black is totally depressing and I don't want anyone expressing their sorrow over my passing," Hartfield's instructions read.
Hartfield asked that songs by Nina Simone and Johnny Cash be played during his funeral.
"I would like for everyone to enjoy themselves. And remember me for who I was. The truth only. None of that stuff about how great I was. Only real stories," he wrote.
Clark County Sheriff Joe Lombardo called Hartfield a "remarkable officer" killed by "an unremarkable person."
A GoFundMe has been set up to help the family.