Well... Have you tried the suggested solution by the system? Namely, turning off thr proxy. Even though you didn't turn it on or you don't even know you had one, it is possible that you once added certain plugin or set a ip-changing app to auto-run on boot up. If this helps, suppose you use Mozilla, i know they have an addons that will change your ip and (at least it happened on mine) it has its own free will. Literally. It turns on and off whenever it sees fit. I ended removing that addons.