is this normal sexual exploration?

We are new foster parents. We are currently caring for an 18 month old little girl. We have had her for 6 weeks now. About 2-3 weeks ago she started hitting, biting, and temper tantrums. All somewhat normal and hopefully a good sign that she feels safe with us. During diaper changes she started to touch herself. I was not concerned because it all seemed normal. (Although she does seem to have a better idea where things are then my girls did at that age.) I just ignore the behavior and continue to change her. One week ago, she smiled at me in a different way, touched herself, then extended her fingers out to me. I ignored this. She then took her fingers and put them in her mouth then repeated. Everthing was very deliberate. Now almost every diaper change she does this although she had stopped offering me the fingers. So, I just have a feeling that this is not normal and I have a sick feeling in my stomach. There was concern that she was sexually abused and I was told her previous caregiver (male) was acting out sexually although there were no further details. Is this something I should talk to the case worker about? Document it in my paperwork? Please help!