I believe we have only ever known of one similar enquiry before and we cannot be sure of the answer. I therefore have only one suggestion at the moment.
When you log in, you can select the length of time to stay logged in. It sounds as though you may have inadvertantly selected the shortest which is 15 minutes. I am not convinced though because the default is 6 hours. However, try selecting "forever" from the drop down menu in the hope that solves the problem.