My 301 redirect disappears

I use a 301 redirect in the htcacess file to redirect from myweb.com to http://www.myweb.com
I don´t know if this problem is related with the pluging settings.
Every 7 days approximately htcacess file is rewritten itself and removes only my redirect 301.
Any idea??

http://wordpress.org/extend/plugins/wp-super-cache/