The girl was sleeping in her Hayward home around 2 a.m. Thursday when a stray bullet from drive-by shooting wounded her in the buttocks, Sgt. Mark Ormsby said.
When the girl woke up in pain and had blood in her underwear, her parents thought she had started her period, police said. They also found no indication she had been shot when they took her to the bathroom.
Ormsby said when she woke up for school and was still in pain, her parents inspected her bed and saw bullet holes.
The girl's doctor told investigators the bullet's entry wound was small and could have been easily overlooked if the family thought she was menstruating.
The girl remained hospitalised in stable condition Friday, Lt. Mark Stuart said.
Police don't have a motive for the shooting and no arrest has been made.