insert command denied in mysql

I have a database table from which, when I select data with username and password, it works but when I insert some data in that table with same username and password it shows me the error

INSERT command denied to user 'username'@'localhost' for table 'tablename'

I searched using Google and someone else had the same problem with db size. I checked my db and when I go directly to phpmyadmin to insert data into the table, it works, but when I call a query to the table from a PHP script, it gives me the error.