For education reasons you should google : "php allowed memory size exhausted". This is typically set in your php.ini. file, in-which can be raised if your host gives you access to it.
It can also mean you have a memory leak in one of the MODS you are running.
See
... "memory_limit = `128M"
see ... "ini_set('memory_limit','128M');"
http://php.net/manual/en/ini.core.php
I have seen this occur when users have many styles. Do a little research before you play with these numbers.