This happened to me in my early twenties. I'd have some symptoms, look them up, and it always led to either cancer or aneurysm. It actually made me a hypochondriac for years, and I had to just stop.
The thing to realize is that the map between illnesses and symptoms is many-to-one. Given some symptoms, doctors assume the most common thing (because then they're right most of the time) and WebMD assumes the scariest thing (because it benefits their traffic).