She is trying to bury something. Tina does this too. It took us a few months to figure out what she was up to. She tries to bury her excess food, toys she doesn't want Tbone to have, or anything that she doesn't like. If something smells funny, she wants to bury it. I love the way her hair looks when she's done.
Sometimes she will find a toy or a sock and use it to bury whatever it is she is trying to hide. I don't think it has anything to do with going into heat (but I do think you should have her spayed before the first heat!)