Hi, there are a few reasons why she might be digging. If the temps are too high, they try to burrow down to escape the heat. So, make sure you are using either a temperature gun or a digital thermometer with a long wire & probe end to check them as any type of stick-on can be as much as 20 degrees off, sometimes more.
Another reason is that she could have some unfertilized eggs (if never with a male) and could be getting ready to lay them. How has her appetite been, does she seem uneasy? It would be a good idea to put a laying box in her tank just in case.