Post keep updating

Hi,

I posted one post on Tuesday 9 Aug, Wednesday 10 Aug and Thursday 11 Aug of last week. Since then the post have up-dated themselves without any help from me as follows: The Tuesday post on 9 Aug was posted on 10 Aug along with the Wednesday post, so each day since then all posts have moved to one day later in the week. Am I correct in thinking that a post should remain on the date it was published? I am using theme Twenty Eleven 1.2.