Session timeout is quicker than i expecting IN ASP 3.0. Please, take a look around

Hi all,
I am using ASP 3.0 for creating a simple web site.
I write a session for login name: Session("MM_Username").
Pages, have to be restricted from unlogged users, check the
MM_Username session's value whether a user logged in. The session
timeout is 100 minutes for the web site I created in IIS 5.1.
But the problem is: the session is expired not in 100 minutes. It is
expired just in 2 or 3 minutes, it is not explicit and it seems to be
not relevant to the timeout in IIS.
Also, I have tried Session.timeout = 100. But there is nothing has
changed.

I did nothing about removing session contents on pages.

Help me, please, I need the expiring of session in minutes chosen by
me. What is happening?

.... Orgil