Custom membership provider

I implemented custom membership provider by deriving from MembershipProvider class. I added login control to my web page and associated my membership provider with my custom database, where I`m keeping all users. This works great.

Whats more, I would like to enable 'Change password' scenario by using the standard ASP.NET control. I created dedicated web page for it and put this control onto form. When I`m willing to change my password (after I logged in to my web application), GetUser(string username, bool) method is being called from my custom membership provider. It`s cool, but why I`m getting my OS login name instead of the login which I used to log onto my web site? In other words: assume that I logged into my web application with {user: User1, password: qwert} credentials. I expect that GetUser method will pass 'User1' as a first argument. Currently it passes '[domainName]/[WindowsLoginName]'.

I believe there is some magical switch in IIS (I`m using v7.5) or web.config file. Could you please help me?