This has been bugging me, so I went in and talked to my Physiology professor who kind of clued me in a little bit on the lingo, and vouched for the wiki on this topic. My doctor isn't alarmed by these raised levels, but I'm not entirely sure about her sometimes so when your doc took it so seriously I got a bit worried. Anyway, here's some stuff to ask your doc about.
From the wiki:
Significantly elevated levels of ALT often suggest the existence of other medical problems such as viral hepatitis, congestive heart failure, liver damage, biliary duct problems, infectious mononucleosis, or myopathy. For this reason, ALT is commonly used as a way of screening for liver problems. However, elevated levels of ALT do not automatically mean that medical problems exist. Fluctuation of ALT levels is normal over the course of the day, and ALT levels can also increase in response to strenuous physical exercise.
When elevated ALT levels are found in the blood, the possible underlying causes can be further narrowed down by measuring other enzymes. For example, elevated ALT levels due to liver-cell damage can be distinguished from biliary duct problems by measuring alkaline phosphatase. Also, myopathy-related ALT levels can be ruled out by measuring creatine kinase enzymes.
~hugs~