My baby is sucking on her lip to the point where it starts bruising... Caused by teething?

My daughter is 8 months old and the other day I noticed that she was sucking on her lip, and at first I thought it was really cute until I realized that she was doing it so much that her lip was bruising. This got me a little worried so I called her doctor and she said it was a rare thing that some babies do while teething. Did anyone else have this? Or think that it could/would be something else???