potty training

Dear all, I have a question. My daughter is 2 and 1/2 yrs old, ( and
is very forward in speach and development ) and is pretty good at
going to the toilet to do a wee, but takes herself away and hides to
do a poo, wipes handfuls on the carpet, then wipes herself on a
pillow, or a book or a soft toy etc. she knows this is wrong. At
first I ignored it as a mistake, then realised that she knew it was
wrong and smacked her, but now I have resorted to putting her back in
full time nappies and putting clothes on her that she can't get into.
Why is she doing this? (I have 2 older girls, but had nothing like is
) PLEASE HELP ME !!!!!!! any suggestions? thanks!!!