Hi- I have had a similar experience with my female black and white tegu in the fall. She has strong hibernation instincts and prepares herself in the fall for her winter's dormancy period by fasting. After several weeks or even a month of not eating, she will finally realize that winter is not coming this year, and she will eat.
the red tegus and the black and white tegus come from a part of the world with cold winters, and they have therefore evolved this hibernation period to survive untill spring. If everything else is O.k. with your tegu, it is probably nothing to worry about. It will eat when it decides it is not about to get cold and dark.