In my limited experience I've come to the conclusion that you can only trust a doctor to diagnose the most common diseases. Most GPs aren't even well versed in cancer incidence, and will dismiss cancer symptoms as something else, unless the evidence is overwhelming. But it's understandable that, when presented with a few symptoms, a doctor will be biased towards common affectations.

That leaves the burden of proof on yourself. If you feel that something is not right with your body, pester your doctor until you get a satisfactory solution. Unless you're a hypochondriac, that is.