Colitis diagnosis procedure

Hello all,

I was diagnosed with ulcerative colitis (pan colitis) after being mis-diagnosed with gastroenteritis.  I was suffering with UC for about 3 months until I underwent a colonoscopy.  

I was first diagnosed with gastroenteritis after undergoing a sigmoid colonoscopy (I was given no medication), and after two weeks all the symptoms were still there.  I was given no medication as the GI doctors said the symptoms should go away on their own.  The GI doctor told me that they would have to wait for the condition to "develop" to change the diagnosis despite the fact that I was suffering for over 2 weeks at this point and was losing weight and unable to perform at work.  They were still saying it could be just gastroenteritis despite having the condition for well over the 2 week period that is common for gastro.  I went to the ER 3 times and each time I was turned away after given IV (for fluid replacement), and told to wait on the GI.

The question I have is, is it a common GI practice to make someone wait for 3 months for the colitis to "develop" before doing a colonoscopy to determine whether UC is present?