I think it is important for you to find out exactly what happened. It might be a good idea for her to see a counselor. Sexual behavior from an 8 yr old is not normal. These things have tremendous impact on young lives and can negatively impact their future if not dealt with correctly.
Kids are starting out very young these days.my cousin was like 8 when girls called him saying what they wanted "to do" to him.which he thought waa nasty.you need ask her more questions...was she forced?did she want to do it?who was it? Where? It seems she's embarrased or something cause she held it in for 3 yrs ask her did she go all the way.what did they do? And why didn't the doctor pick this up? The doc not picking it up leads me to thinking maybe it was oral (srry) or something ....
This could just be exploratory behavior or it can be more than that. It is hard to tell given your limited description of the issue. If you suspect that this issue is more than exploratory behavior, it might not be a bad idea to have your daughter evaluated by a mental health professional. The school counselor might be able to assist with this.