Things to look at:
1. Are you using a plugin that lets you change it? One of our favorites around here is Custom Query String. If you’re running this, the admin panel for it will be under Options > CQS.
2. Are you using query_posts() or an equivalent to generate the posts list? This can be checked by looking at the source of the theme template used for your categories.
Problem here is, we won’t necessarily know *which* template is the specific one to look at, because a feature known as ‘template hierarchy’ lets a query type (say category) inherit various templates depending on which ones are available in a theme.
I won’t go into the technical aspects of this (because they’re boring!), so let’s cut to chase… I did a bit of ‘dipping’ in your active theme’s directory, and found you have the standard Category template (category.php). So that’s a good place to start.
If you need help looking over your category.php, you can paste up the source here:
https://wordpress.pastebin.ca
and reply back with the url to it.
EDIT: This thread really doesn’t belong in Installation, so I’m moving it to How-To and Troubleshooting…