I am getting a "Error writing file '/tmp/MYW0OfVN' (OS errno 28 - No space left on device)" but my website is 100mb, MySQL DB is 10mb and disc is 40gb

my website will not open due to "no space left" seems like a MySQL temp issue, but how and why is it creating a 40gb temp file. - the MySQL database is 10mb only

Can anyone please help get rid of this huge temp file and let me know how to stop it happening again