What Does Blood Spots In Urine Indicate In 12 Years Old Girl?

my 12 year old granddaughter has a small amount of blood in her urine. It isn't visable to the naked eye. She had a stomach ache several weeks ago. Her urine was tested then and there was a small amount of blood. Same with the test today. She has not had any symptoms of anything and her stomach ache is gone. The dr said to come back in 2-3 weeks. What could it be?