Eleveted alt level

Hi, I am a healthy 40 years old female, I regularly exercise and don't drink alcohol. After doing my routine blood test, my doctor find eleveted alt level (127). She did test for epatites which were negatives and an abdomen ultrasound . No abnormalities were found, the liver and all the other organs were normal. I don't take any medications and she can't figure out why such high levels of liver enzimes were present. She told me to wait 2 months and to repeat the test. What could it be?