It seems there are a lot of ways for UC to begin. I had a pretty severe, sudden onset shortly after I had my son. I've heard stories on here of some people who suspect they had mild symptoms for years before being diagnosed.
Your current symptoms sound very much like an IBD (UC or Crohn's) to me. They only way to really tell, though, is to have a colonoscopy.
It's probably good your PCP wouldn't give you more pred. It has some pretty serious side effects that come from long term use and if you're on it when you have a colonoscopy and you do have UC, it might have the inflammation (that is a key diagnostic factor) under control, resulting in a misdiagnosis.
Let us know what the GI says and welcome to the forum!
Symptoms began in November 2008, ~4 weeks after giving birth to my son.
Eased for ~3 weeks in December, possibly b/c of probiotic use?
Returned in January 2009 (with a vengeance), diagnosed with pancolitis on January 30.
Currently taking Asacol (400mg 4 pills 3x daily), Rowasa nightly, Culturelle probiotic, and Zoloft (25 mg).