CD can be very difficult to diagnose. I was diagnosed with IBS two years ago, and then CD last month. So yeah, it seems pretty common.
To get a diagnosis of Crohn's, my doctors told me you need to have a few things present. This is probably not very accurate, and I'm sure many people have been diagnosed with CD without having all of these symptoms present.
Here is was I presented with, and what helped with my Crohn's diagnosis.
-Anemia. Low iron, shown through blood work.
-Elevated white blood cells. This shows infection or inflammation somewhere. Also shown through blood work.
-Bloody diarrhea. It doesn't have to bloody all the time, but sometimes.
-Frequent diarrhea. I was going up to 25 times a day, hence my username lol
-Abdominal pain. Usually in the lower right quadrant. It can be sudden onset severe pain, or constant, dull aching pain. I had both.
-CT scan, showing inflammation, usually in your small bowel and at the terminal ileum.
-colonoscopy, showing ulceration, inflammation, swelling, bleeding or "cobblestoning."
You could also have joint pain, eye inflammation, mouth ulcers (canker sores,) weight loss, loss of appetite, apparent food sensitivities, open leg sores.
I have all of these, except the leg sores. But it took me over ten years to get a diagnosis. I was told I had dozens of other medical problems, which I don't have. They have to be able to see the inflammation in your intestine, before they can give a hard diagnosis of crohn's. You should try to get a colonoscopy while your symptoms are at their worst. I went to the emergency room while my symptoms were really bad, and was diagnosed within 2 days. Hope this helps!