I think you have plenty of thumb sucking advice, so I will touch on the other. Let me first start by telling you that when my daughter was 1 or earlier...whenever she could talk, we learned our parts...and what was appropriate and not appropriate and who can and who cannot do whatever...and to always come to mommy with ANYTHING relating to a touch or attempt, or sketchy behavior from another.
My daughter is 6 now and is very vocal to me about how good it feels and how it tickles to touch it. She asks why it feels tht way and I tell her because God made it that way and that it is supposed to. Remember when they were babies, they did the same thing then. Self soothing in the private area. It is perfectly natural. For a child it is not a sexual thing, but it just feels good.
I read how someone suggested talking to her about had anyone touched her inappropriately. That is a good idea, but I wouldn't discuss the 2 on the same day, so she doesn't think you are telling her she is doing something bad. I think you might tell her (after finding out nothing has ever happened to her that should not have) that it is okay to touch herself there, but only in private. In her room alone, in the tub alone and certainly not to discuss that with anyone other than yourself. That she, of course, isn't to touch anyone else there and vice versa.
Bottom line, it is natural, and not an indicator that there has been any molestation. Let her sooth herself...and give her a lullaby CD so she has a variety of soothing things....
Good luck, it will all be just fine!
A.