They probably have a quota rule setup…i.e. the monitoring software can email the system administrator when an employee spends more than x number of minutes on a specific site (with obvious exceptions). So, keep your daily and weekly total visit time below 10 minutes or so a day and your probably fine.
I should also caution you that with monitoring programs, they will know what specific individual pages you visited, what time you visited, and how long you spent surfing. The IT department has a detailed log of all this and it could easily be forwarded to your supervisor.
Ask someone else you work with to try accessing the site for their computer. It is likely that your IT department is using SurfControl or a similar web filter monitoring and blocking software. The only possible method for circumventing this system is by attempting to access “cached” pages via Google or using a client like AOL to access those sites. You can also attempt to make “friends” with the IT department to get your access back. Good luck.