Jamie Gilt, 31, was taken to a hospital after the Tuesday shooting, and is believed to be in stable condition.
A Putnam County Sheriff's deputy was on patrol when he noticed a truck stopped partially in the travel lanes and a woman in the driver's seat motioning for help. The only other occupant in the vehicle was the boy, who was not harmed.
"She was sitting in the driver seat and he was in the back seat, behind her. He shot straight through. The bullet entered her lower back and exited through her abdominal area. It went through her and we recovered the round inside the vehicle. It was a .45 caliber (handgun)," Capt. Joseph Wells said.
Authorities said the firearm was legally owned by Gilt, who maintained a Facebook page entitled "Jamie Gilt for Gun Sense," where she regularly posted pro-gun positions. On her personal Facebook page, Gilt once bragged about her son: "Even my 4 year old gets jacked up to target shoot with the .22."
Both pages appeared to have been taken down by Wednesday afternoon.
Detectives have not yet been able to interview Gilt because of her medical condition.
Florida law makes it a misdemeanor for a person to store or leave a loaded firearm in such a way that a child could gain access to it.
Authorities said they won't make a decision on charges until they can speak with the woman.