As Keeper says, c-diff is clostridium difficile. It normally happens when people are on antibiotics, and these antibiotics upset the normal flora of the gut, allowing c-difficile to take root. C-diff is a spore forming bacteria that can live on surfaces for months, and once you ingest these spores, they can lie in wait for an opportune moment in your gut to take hold. Also, untreated c-diff can result in a severe infection that causes pseudomembranous colitis. C-difficile infection is characterized normally by large watery bowel movements, with lots of frequency, and a very distinctive smell.
However, antibiotics are not the only way to end up with a c-diff infection. My husband has been on no antibiotics, but has now had a c-diff infection twice! They say that people with IBD like Crohn's and UC are more susceptible to infection with c-diff, because IBD is a disease that affects the flora of the gut. So, when you have IBD, you can get c-diff even without exposure to antibiotics.
If you suspect you have c-diff, you should be tested for it (stool test), which is simple enough. The test is usually a toxin test that tests the stool for both Tox A and Tox B (make sure that it tests for both toxins). If the test is positive, you can assume that the test is correct, because it has high specificity. However, if the test is negative, and you still have symptoms, get 2 more stool tests, because the test isn't very sensitive, ie. you can get false negatives. The rule of thumb is that if you have 3 negative stool tests from stool taken on different days, then you can assume you don't have a c-diff infection and look for other causes. Also, if you have a c-diff infection, and your infection is severe enough, a colonoscopy will reveal pseudomembranous colitis.
However, it is uncommon to get c-diff without antibiotics - my husband has just been (un)lucky (twice)
It sounds like you need further examination - blood tests for inflammation are not always accurate. It sounds like you need to be tested for any infections, and if no infections are found, treated for a flare up of Crohn's disease, with something stronger than pentasa. A steroid taper perhaps. Go see your GI as soon as you can, so you don't suffer with this for too much longer.
Husband with Crohn's
Diagnosed March 2003 Ulcerative Proctitis
Diagnosed March 2008 Crohn's & C-diff, hospitalized 45 days
Canasa, Lialda, Remicade, VSL#3, Florastor
In Remission since June 2008
Stopped vancomycin for c-diff Jan 1 2009
C-diff free, until Sep 2, 2009
Fighting c-diff, I guess for life