My baby is 2 months old. Her AST level went from 87 to 100 in two weeks. Ultrasound showed enlarged liver but nothing serious. What else can this be?

Many possible causes. There are a number of possible causes of enlarged liver and elevated liver enzymes, ranging from certain metabolic disorders or infections to structural problems. Evaluation usually includes blood tests, ultrasound, seeing a liver specialist, and sometimes other scans or biopsies.
See your doctor. Symptoms like these are best deterimed by a thorough evaluation by your baby's physician. Based on his/her findings, a treatment plan can be developed to help your baby.