You have developed a phobia problem. There's probably no explanation for it. I would look for a psychologist who practices a form of therapy called CBT as soon as possible which will hopefully teach you tools to overcome this. I would do it soon because phobias have a tendency in many people to start spreading as you start avoiding the things that trigger the anxiety. I've had this problem for a long time, and for most of us who do there really is no rational explanation either for why it exists or why the things that trigger it do it and other things don't. Just one of those random things in life.