Yes, it's wonderful that there are reliable medical resources on the web. But it is not wonderful to diagnose yourself via these resources, then argue with your doctor's diagnosis, which was made via examination, testing, and, you know, actual medical knowledge.
Don't be a cyberchondriac. Here's how:
Seek Information, Not a Diagnosis
If you are having weird health issues, go ahead and look them up on authentic websites. You can turn toward sites like MayoClinic, WebMD, MedlinePlus and the National Institutes of Health.
Do not go into this expecting to find the final answer to your issue - at best, it may help clarify what's going on and connect your current symptoms to other things you may not have thought of. This can facilitate a better conversation with a doctor in real life.
When you land on a page about your symptoms, read the "Causes" critically. Don't ignore possible causes like caffeine, insufficient sleep and acidic foods and immediately jump on TUMOR! OMG!! Because it's probably not. The reason these reputable sites are reputable is because they are thorough - they list all the possible causes. Now if only they would list the actual probability of each cause...
Don't Argue With Your Doctor
Once you've done your research, go see your doctor. You could always skip the research part and just make the appointment, but the fact that you're reading this speaks to the fact that you won't. So print off your checklists and whatever page has convinced you that you have multiple organ failure...