SA is one possibility...
So is being exposed to someone else who is being sexually abused (all the kids in my son's K class became very aware about the intimate details of BJ's and anal, because one of the kids in the class was being raped/molested...
It's also possible she's just hit adolescence and is starting puberty. Early starters (period at 8 or 9) get the same crazy hormone things that pre-teens and teens do... right along with sex drive...
It's also possible it's merely her extrapolating / copying stuff from cartoons & tv (think about the way Jasmin pretends to seduce Jafaar in Aladdin, just for one example amongst thousands) or other forms of media (take a good look at the covers of the magazines that are at eye level to HER in the grocery store... they usually include seductive poses, and the "women's" mags include things like "50 great sex tips to make him YOURS" "Falling into Bed with the wrong guys AGAIN", "How to get a guy to blah blah blah"...
It's also possible (depending on your religion) that she's acting out stuff she's heard in church... without the deeper meanings / metaphors adults and older children attach, some of the written descriptions are almost kama sutra esque...
It's also possible that (without more details) what she's doing is totally normal and typical. Masturbating (most toddlers figure out this one... it doesn't actually = sex to them since they don't have the hormones or education yet, it just feel good), totally innocent playground kisses, wanting to play "house" (this is where she'd be combining stuff from home & tv) which is also totally innocent...
So there are a bunch of possibilities, although in my exp, it's usually a combo of several.