It could be a number of things. I have two thumb suckers (even though one is way past the age to quit), and they both started almost from birth, but my niece didn't start until she was almost 1 1/2. Basically, she was fine until she got around my kids. I think sometimes it can be a learned behavior. Someone else does it and likes ti so I'm going to try it too. The biggest thing is don't worry about it too much now. You can reinforce not doing it by brining it to her attention everytime you catch her doing it. If she refuses to stop, don't make a scene. Just explain that big girls don't suck their thumbs. Eventually, the reinforcement should help to stop the problem. If you get into the 5yr range and have exhausted all the home remedies for stopping it, then you might want to talk with a dentist and/or doctor because once the permanent teeth start coming in, the habit will misalign them. (mucho $$$$ for orthodontic work, I'm gonna have an ugly bill with 2). Ultimately if your really concerned, talk with her physician.
Debi