Application trying to use wrong user when logging into the database

Hi,

I'm writing a web app using Node.js and pg-promise along with PostgreSQL. I've been following the documentation to
connectto the database found here: 

https://github.com/vitaly-t/pg-promise/wiki/Connection-Syntax#configuration-object

and have all the right data supplied. But instead of using my applications role which I created in PostgreSQL it is
tryingto use my Linux user name "simon" and is failing authentication because of it. I'm just confused at the moment.
Atno point in time have I created a role called "simon" and I have never tried to connect to any of my databases using
therole name "simon", so why is it trying to authenticate as "simon"? 

Sorry if this is a silly question but it has me completely stumped.

Simon.