Membership.GetUser() is returning null when app starts

I've implemented my own AccountProfile class is ASP.net MVC and it works but now I'm running into a strange problem. First off, I'm calling AccountProfile.MyProperty in the Views/Shared/_LoginPartial.cshtml. AccountProfile.MyProperty makes a call to Membership.GetUser().UserName to work. Now, when I "signup" for an account, after I'm logged in, AccountProfile.MyProperty works and renders info into the html page.

However when I stop running, change some code, and launch again, Membership.GetUser() returns null, even though I'm still logged in according to membership.

After the page fails to load, if I navigate to the /Account/Login page, Membership.GetUser() works. After it works, I can then navigate to the index page that didn't work at start, and it works.

My question is why does Membership.GetUser() return null when my page first loads up?

Thanks.