Thx for answering. The problem was : Nothing was at the right place, images, text, the page width was totally different, the white background of the theme became transparent, and the sidebar was I don't know where... this is was I called "a display in a mess".
After emptying all the caches in W3C and in my browser it was ok again but a while later the problem was coming back.
The support in W3C told me maybe I should disable the Minify css function. I'll test it now.
I'll post the result here, in case it could help someone else....
(**WP 3.2.1 / twenty Eleven theme with a child theme / http://www.hazlotopics.com/blog/)