Fatal error: Allowed memory size…: 128mb not enough!

Hi i have this error: “Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 6181851 bytes) in /home/mysite/public_html/wp-includes/wp-db.php on line 951

I asked to my hosting that told me i have 128mb, so what i have to do?

I have this error when i install wp better security. Any idea of what is the problem? I think 128mb should be enogh to install wordpress!!

Thanks to all