What browser are you using? Do you have any security software installed or security options enabled in the browser? I had this problem too until I realized my browser was not saving cookies after the browser was closed.
It is not likely malware, I experienced this same thing a year ago and it affected several computers that are NOT directly connected to each other in any way (one at work, at home, and at a relatives). Same issues. Then oddly the problem simply disappeared without rhyme or reason. Probably a software glitch.