SPANAWAY, Wash. -- A 25-year-old Spanaway woman pleaded not guilty Tuesday to second-degree murder charges in the beating death of her 5-month-old son, reported KIRO 7 Eyewitness News.

Domenique Conway is accused of punching Nathaniel Noel in the head several times and subsequently causing his death, the Pierce County Sheriff's Office said.

According to charging documents obtained by KIRO 7, Conway was changing the boy's diaper on March 5 and became frustrated when he would not stop crying and squirming. Documents said she was also angry because her two other children, ages 2 and 6 years old, were fighting in an adjacent room.

Conway initially denied hurting the infant and then confessed that she "struck Nathaniel twice with her closed fist on the left side of his head," court documents said.

Police said Conway denied noticing symptoms of the boy’s head injury, including lethargy and vomiting over a four-day period and did not seek any medical attention for the boy until Sunday when she called 911 to report that her son was unconscious.

Investigators said that upon deputies’ arrival they determined the boy was dead. The Pierce County medical examiner determined the boy suffered "blunt force trauma to the head," including internal bleeding that was allowed to go untreated until his death.

Conway was arrested on Monday and her two children were taken into the custody of Child Protective Services. Investigators said Nathaniel's twin brother is already being cared for by CPS for an earlier incident for which circumstances are unknown at this time.