Mackenzie will be 3 in June and has started the same thing. It seems to be more when she has no knickers on or I noticed when we were on holidays when she was swimming (maybe the swimmers were annoying her in the pool?). It drives my DH nutty but I just tell him not to worry about it. She seems to touch and then sniff which is grose but I'm not too worried about it - I'm sure she will stop soon. (heres hoping anyway)