It sounds to me like your login user cookie is expiring far too quickly.
On you login page you should have 3 different fields username, password, as text inputs, and remember as a checkbox.
If you already have this then great but ensure that the value for the checkbox is set to 1 when checked, in fact i normally just make this checked with a value of 1, the user can uncheck it if they want.
By setting the checkbox in this way it will correctly store the cookie to remember the user for 30 days or whatever you have set, if you forget to set the remember value to 1 it will auto log users out like what you are experiencing.