Kaylee-Jayde Priest was found dead at a flat in Stonebridge Crescent, Kingshurst, Birmingham, where she lived with her mother, Nicola Priest, on August 9 last year.
Birmingham Crown Court previously heard the youngster was "dead before the (999) call was made" after allegedly being killed by her mother and fellow defendant Callum Redfern, from Dudley.
On Tuesday, Priest told jurors she did not make the 999 call straight away because she was "scared" and "didn't know how to approach it".
Priest, 22, and Redfern, 21, who prosecutors have alleged was in a "close sexual relationship" with the child's mother, are accused of Kaylee's murder, and a separate alternative charge of manslaughter.
Asked who she believed to have been responsible for her daughter's death, the defendant replied: "Cal."
Kaylee, described in court as a "happy child", died from serious chest and abdominal injuries.
Priest claimed she heard "three to five smacks" coming from the bedroom, where the youngster and Redfern had been the night before she was found unresponsive.
She told jurors: "I ran into the bedroom and she was holding her stomach.
"I said 'what the f*** happened'?"
Questioned by defence QC George Carter-Stephenson why she did not think to immediately call 999, Priest said: "I was scared I didn't know how to approach it.
"I thought instantly she was dead.
"I tried to do CPR.
"I was scared to leave her.
"I was trying to do what she asked me.
"I felt sick, upset and heartbroken.
"I had just lost my little girl."
Asked what happened when she found her daughter in the morning of August 9, Priest said: "I just remember dropping to the floor and screaming.
"I could see my daughter had passed and was stiff."
Jurors previously heard claims that Priest would hit Kaylee around the head and refer to her as a "f****** brat", while the youngster was also heard crying "in a fearful tone".
Mr Carter-Stephenson asked the defendant if she would have agreed to someone inflicting the injuries on her daughter identified by medical experts, to which she replied: "No.
"If I had known that punching or kicking had been done I would have taken her straight to the hospital, if I had known."
Priest, of Poplar Avenue, Edgbaston, and Redfern, of Temple Street, Dudley, are also accused of causing or allowing the death of a child and cruelty to a child, between June 12 and August 3 2020.
They deny any wrongdoing and the trial continues.