I have a VDS on Ubuntu 10.04.4. It works as HTTP server (Apache, Nginx, MySQL, PHP, Postfix), no FTP, root log in disabled. Suddenly, my VDS was blocked, because exceeded traffic quota (300 GB a day!). I have bought a few GB and check logs - nothing strange isn't found, no log ins, no big quantity requests to the web servers. Next day I observed traffic and it was normal, last check was at 19:00. At 23:50 server was shutdowned again, because it again eaten all traffic.
What can I check, or install to find problem?