Mom of Toddler Allegedly Murdered by Dentist Boyfriend Details Moment She Realized Son Was Dying

Oklahoma Dentist Bert Franklin, a married father of four, allegedly dropped his girlfriend's son, Lincoln, on his head before kicking him, police have said

The Oklahoma mom whose 19-month-old child was allegedly killed by her dentist boyfriend is speaking out about her loss in an upcoming episode of Dr. Phil.

Roxanne Randall’s toddler son, Lincoln, died in July, two days after her then-boyfriend Bert Franklin — a married father of four — allegedly dropped him on his head and kicked him, police said.

Franklin has since been charged with first-degree murder in connection with Lincoln’s death, court records show. He pleaded not guilty and remains behind bars without bond. His attorneys did not return a Friday message seeking comment.

In an exclusive clip from her sit-down with Dr. Phil McGraw, airing Monday, Randall describes the moments she realized something was wrong with her son.

“I woke up at [2 a.m.] to a sound; I thought Lincoln was coughing,” she details to McGraw. “I woke up and I just remember instantly thinking, ‘Oh, I knew it’ — he was sick.”

Randall says she went to the kitchen to get Lincoln some medicine and his bottle, and when she returned the toddler was “gurgling.”

“I put the stuff down and I grabbed him. And when I grabbed him, his body was limp,” she says. “So I yelled at [Franklin] to call 911, and he stood up at the other side of the bed and just stared at me.”

A source close to Randall told PEOPLE last year that she knew there was a problem on the day after the alleged incident, when Lincoln — his eyes wide open — started having trouble breathing.

Part of Franklin’s alleged abuse of Lincoln was captured by a home surveillance camera. The footage does not show the entire incident, however, and Franklin’s attorneys have said he was kicking a dog and not the child.

• Want to keep up with the latest crime coverage? Click here to get breaking crime news, ongoing trial coverage and details of intriguing unsolved cases in the True Crime Newsletter.

In the surveillance video, Franklin can be seen cradling a limp Lincoln while eating a slice of pizza — all following the alleged altercation.

Randall was home at the time, but told investigators she was upstairs. She claimed that she ran to the basement after hearing a loud thud, but Franklin told her the noise came from him throwing a ball to the dog.

According to medical examiners, Lincoln suffered a skull fracture and brain bleed. He was pronounced dead after being hospitalized.

The Randall source previously told PEOPLE that she and Franklin had been dating for nearly a year, but Randall didn’t know he was still married.

“He was living a complete lie,” the source said.

The Dr. Phil interview with Roxanne Randall airs Monday. Check local listings.

• With reporting by ADAM CARLSON